Why pay 20 dollars for this when you could pay 6 for GMod? Just saying.
Many reasons, actually;
One being, Gmod is not $6.
It's $10. And Gmod can't run by itself, it is a source game. Thus you need to invest in Counterstrike, Team Fortress 2, or the best supporter, Half Life 2. This could wrack up to about $30. You could get deals like Half Life + Counterstrike + Garrysmod for say, $20.
In the end Gmod costs more than $6, more than even it's own price.
Blockland is $20. Done deal, pretty much (unless you're glass than it's like $400)
And it's all about likeness. Would someone play Garrys mod or Blockland for, physics? Characteristics, playertypes. I personally would take both, as I've done. I just don't have a computer that can run Garrys mod the fanciest.