For one, Pinkie Pie wasn't trying to kill the friendship between Rainbow Dash and Gilda, she wanted to be friends with both of them.
And secondly, it was Gilda who indirectly called Rainbow Dash lame, and then assumed that nobody wanted to forgive her and stormed out of the party.
Let's go through the transcript shall we?
Pinkie Pie: Gilda! I'm so honored to throw you one of my signature Pinkie Pie parties, and I really truly sincerely hope you feel welcome here amongst all us pony folk.
Gilda: [grunt]
Pinkie Pie: [laughs]
[laughter]
At this point, Gilda realizes that Pinkie Pie is obviously planning more pranks, having just got humiliated by some kind of high-voltage joy buzzer in front of a crowd composed of every pony in Ponyville, and is trying to act cool even though she knows that they're all in on it.
Rainbow Dash: Oh Pinkie Pie, the old hoof-shake buzzer. You are a scream.
Gilda: Yeah [chuckles nervously] uh, good one, Pinkie Pie.
Gilda, trying to be a good sport for her friend.
Rainbow Dash: Come on G, I'll introduce you to some of my other friends.
Gilda: Right behind you Dash. I know what you're up to.
Pinkie Pie: Great.
Gilda: Rrrh. I know what you're planning.
Pinkie Pie: [giggles] Well I hope so. This wasn't supposed to be a surprised party.
Gilda: I mean, I've got my eye on you.
Pinkie Pie: And I got my eye on you.
Pinkie Pie, trolling the forget out of Gilda, who's only still at the party because she's trying to please Rainbow Dash.
Pinkie Pie: Everyone, I'd like you all to meet Gilda, a long-time dear friend of Rainbow Dash. Let's honor her and welcome her to Ponyville.
[ponies cheer]
Pinkie Pie: Please help yourself.
Gilda: Vanilla lemon drops. Don't mind if I do. HOT!
Rainbow Dash: G, the punch.
Gilda: [gasp] [gulp] Huh?
[laugher]
Pinkie Pie: Well whaddya know, pepper in the vanilla lemon drops, and the punch served in a dribble glass.
Adding insult to injury, having just humiliated Gilda twice more in a party that EVERYONE in Ponyville is attending.
Rainbow Dash: Ha. Priceless. Priceless. [laughs]
Gilda: [gulp] [gasp] Yeah, hilarious.
Trying to keep her cool, obviously extremely pissed off.
Rainbow Dash: Hey G, look, presents.
[laughter]
Applejack: Spittin' snakes. Heh, somepony pulled there prank on me last month.
Gilda: Ha ha. I bet I know who that was.
Pinkie Pie: You do?
More trolling from Pinkie Pie, Gilda pissed as all hell but still keeping somewhat of a cool.
[ponies chatter]
Pinkie Pie: Cake time everypony.
Spike: Hey, can I blow out the candles?
Twilight Sparkle: Why don't we let Gilda blow out the candles, Spike. She is the guest of honor after all.
Spike: Ugh.
Gilda: Exactly. [gasp] [blow] [gasp] [blow] [gasp] [blow]
[laughter]
Gilda, trying to play along gets pranked, again with the entire population of Ponyville there to see it.
Gilda: [gasp] [blow] [gasp] [blow] [gasp] [blow] [gasp] [blow] [gasp] [blow] [gasp] [blow]
Spike: Re-lighting birthday candles, I love that prank. What a classic.
Pinkie Pie: Now, I wonder who could've done that.
Gilda: Yeah, I wonder.
More trolling from Pinkie Pie.
Spike: Mmm, who cares. This cake is amazing.
Twilight Sparkle: Spike.
Spike: What? It was great, try some.
Rainbow Dash: Hey G, you're not upset about some silly candles, are you?
Gilda: No way Dash, like I said, I'm down with a good prank.
Rainbow Dash: Come on then, let's have some cake.
Gilda, trying to give the impression that she likes Dash's friends who are being total starfishs to her.
Gilda: Hey, I'm watching you. Like a hawk.
Pinkie Pie: Why? Can't you watch me like a griffon?
Trolling.
Applejack: Hey y'all, it's pin the tail on the pony. Let's play.
Rarity: Oh, my favorite game. Can I go first? Can I have the purple tail?
Gilda: Well I am the guest of honor, I'll have the purple tail.
Pinkie Pie: Yeah, Gilda should definitely go first. Let's get you blindfolded.
Gilda: Hey what-- ugh-- what are you doing? Rrrah.
Pinkie Pie: We're spinning you around and around and then you pin the tail on the pony. Now just walk straight ahead and pin the tail.
Gilda: [mocking: Now just walk straight ahead and pin the tail.] Heh, yeah right, this is another prank isn't it. I'm going this way.
Pinkie Pie: Wait. The poster is this--
Gilda: Whoa whaa waah waah waaaaah [groan]
Pinkie Pie: Uh, Gilda? You pinned the tail on the wrong end.
[ponies laugh]
After the seventh time she's been utterly humiliated in front of everyone in Ponyville, she snaps.
Gilda: [roar] This is your idea of a good time? I've never met a lamer bunch of dweebs in all my life. ...and Pinkie Pie, you, you are queen lame-o with your weak little party pranks, do you really think you can make me lose my cool? Well, Dash and I have ten times as much cool as the rest of you put together. Come on Dash, we're bailing on this pathetic scene. Come on Rainbow Dash. I said, we're leaving.
Gilda raging, and for good reason. All this time she's expected Pinkie Pie of trying to destroy hers and Dash's friendship, and now she's positive that Pinkie Pie had orchestrated this entire thing to make her lose her cool and bitch out everyone there.
Rainbow Dash: You know Gilda, I was the one who set up all those weak pranks at this party.
Gilda: [gasp] What?
Pinkie Pie: Ooh.
Rainbow Dash: So I guess I'm queen lame-o.
Gilda: Come on, Dash, you're joshing me.
Rainbow Dash: They weren't all meant for you specifically, it was just some luck that you set them all off.
While Rainbow Dash set the pranks, Pinkie Pie manipulated Gilda into falling for all of them. Gilda really doesn't want to believe that Rainbow Dash and Pinkie Pie had organized these scenes of utter humiliation in front of the entirety of Ponyville, because a good friend would never do something like that.
Pinkie Pie: I shoulda known, that dribble cup had Rainbow Dash written all over it.
A blatant lie, she knew where all the pranks were, because that's how she tricked Gilda into triggering every single one of them.
Gilda: No way. It was Pinkie Pie, she set up this party to trip me up, to make a fool of me.
Pinkie Pie: Me? I threw this party to improve your attitude. I thought a good party might turn that frown upside down.
Another lie, Pinkie Pie greeted Gilda with a joybuzzer, it was an attempt to piss her off in front of Dash from the start.
Rainbow Dash: And you sure didn't need any help making a fool of yourself. You know, this is not how I thought my old friend would treat my new friends. If being cool is all you care about, maybe you should go find some new cool friend someplace else.
Gilda: Yeah? Well you, you, you are such an, a flip-flop: cool one minute and lame the next. When you decide not to be lame anymore, gimme a call.
Gilda has a very good point, she gave the false impression that she cared about Gilda at all, and she's a bitch for it.
[ponies commentate: Wow, what was that about? / Awkward.]
The victory goes to Pinkie Pie.
Poll