Are you adding the add-on to the gamemode.txt file in the gamemode folder? Or just to your add-on folder.
If you want to edit the speedkart gamemode then you'll have to make a copy and it and rename it. zip files take priority over folders(?) and if you modify the file and then zip it again, it'll be overwritten when you launch Blockland.
//Default stuff...ADDON Brick_Arch ADDON Brick_Large_Cubes ADDON Support_Player_PersistenceADDON Brick_Treasure_Chest ADDON Brick_V15 ADDON Emote_Alarm ADDON Emote_Confusion ADDON Emote_Hate ADDON Emote_Love ADDON Light_Animated ADDON Light_Basic ADDON Particle_Basic ADDON Particle_FX_Cans ADDON Particle_Player ADDON Particle_Tools ADDON Particle_GrassADDON Player_No_Jet ADDON Print_1x2f_DefaultADDON Print_2x2f_DefaultADDON Print_2x2r_DefaultADDON Print_Letters_Default ADDON Sound_Beeps ADDON Sound_Phone ADDON Sound_Synth4 ADDON Vehicle_Pirate_Cannon ADDON Vehicle_TankADDON Weapon_Rocket_LauncherADDON Projectile_GravityRocketADDON Vehicle_Jeep ADDON Vehicle_HorseADDON Vehicle_Rowboat ADDON Weapon_Bow ADDON Weapon_Spear ADDON Weapon_Sword ADDON Support_DoorsADDON Brick_DoorsADDON Brick_HalloweenADDON Brick_TeledoorADDON Brick_Christmas_TreeADDON Item_Boombox//Kart racing stuff...ADDON Brick_ModTer_BasicPackADDON Brick_ModTer_InvertedPackADDON Brick_Modter_4xPackADDON Print_ModTer_DefaultADDON GameMode_SpeedKartADDON Server_VehicleGoreADDON Event_Camera_Control//botsADDON Bot_HoleADDON Bot_BlockheadADDON Bot_HorseADDON Bot_SharkADDON Bot_Zombie//for banditoADDON Weapon_GunADDON Weapon_Guns_Akimbo//Music for BOOMBOXMUSIC After_School_SpecialMUSIC Ambient_DeepMUSIC Bass_1MUSIC Bass_2MUSIC Bass_3MUSIC CreepyMUSIC DistortMUSIC DrumsMUSIC FactoryMUSIC IcyMUSIC JungleMUSIC PeacefulMUSIC Piano_BassMUSIC RockMUSIC Stress_//environment$EnvGuiServer::SimpleMode 0$EnvGuiServer::SkyFile Add-Ons/Sky_Skylands/Skylands.dml$EnvGuiServer::WaterFile Add-Ons/Water_Brick/brick.water$EnvGuiServer::GroundFile Add-Ons/Ground_Plate/plate.ground$EnvGuiServer::SunFlareTopTexture base/lighting/flare2.png$EnvGuiServer::SunFlareBottomTexture base/lighting/corona2.png$EnvGuiServer::DayOffset 0$EnvGuiServer::DayLength 300$EnvGuiServer::DayCycleEnabled 0$EnvGuiServer::DayCycle Add-Ons/DayCycle_Default/default.daycycle$EnvGuiServer::Sunational socialistmuth 238$EnvGuiServer::SunElevation 21$EnvGuiServer::DirectLightColor 1.000000 0.915888 0.836449 1.000000$EnvGuiServer::AmbientLightColor 0.612150 0.592126 0.592126 1.000000$EnvGuiServer::ShadowColor 0.000000 0.498211 0.579439 1.000000$EnvGuiServer::SunFlareColor 0.200000 0.200000 0.200000 1.000000$EnvGuiServer::SunFlareSize 2.25641$EnvGuiServer::VisibleDistance 753.744$EnvGuiServer::FogDistance 471.795$EnvGuiServer::FogHeight $EnvGuiServer::FogColor 0.771028 0.826666 1.000000 1.000000$EnvGuiServer::WaterColor 0.172897 0.630841 1.000000 0.827103$EnvGuiServer::WaterHeight 5$EnvGuiServer::UnderWaterColor 140 178 255 134$EnvGuiServer::SkyColor 1.000000 1.000000 0.785047 1.000000$EnvGuiServer::WaterScrollX 0$EnvGuiServer::WaterScrollY 0$EnvGuiServer::GroundColor 1.000000 0.816578 0.607477 1.000000$EnvGuiServer::GroundScrollX 0$EnvGuiServer::GroundScrollY 0$EnvGuiServer::VignetteMultiply 0$EnvGuiServer::VignetteColor 0.000000 0.000000 0.000000 0.392157//minigame rules$MiniGame::Enabled 1$MiniGame::GameColor 0$MiniGame::InviteOnly 0$MiniGame::IncludeAllPlayersBricks 1$MiniGame::PlayersUseOwnBricks 0$MiniGame::Points_BreakBrick 0$MiniGame::Points_PlantBrick 0$MiniGame::Points_KillPlayer 1$MiniGame::Points_KillBot 0$MiniGame::Points_KillSelf -1$MiniGame::Points_Die 0 $MiniGame::RespawnTime -1$MiniGame::VehicleRespawnTime -1$MiniGame::BrickRespawnTime 60$MiniGame::BotRespawnTime 5$MiniGame::UseSpawnBricks 1$MiniGame::FallingDamage 1$MiniGame::WeaponDamage 1$MiniGame::SelfDamage 1$MiniGame::VehicleDamage 1$MiniGame::BrickDamage 1$MiniGame::BotDamage 1$MiniGame::EnableWand 0$MiniGame::EnableBuilding 0$MiniGame::EnablePainting 0$MiniGame::PlayerDataBlockName No-Jet Player$MiniGame::StartEquipName0 0$MiniGame::StartEquipName1 0 $MiniGame::StartEquipName2 0$MiniGame::StartEquipName3 0$MiniGame::StartEquipName4 0$MiniGame::TimeLimit 240//server settings$Server::Quota::Schedules 9999$Server::Quota::Misc 999$Server::Quota::Projectile 999$Server::Quota::Item 999$Server::Quota::Environment 9999$Server::Quota::Player 100$Server::Quota::Vehicle 50$Server::MaxPhysVehicles_Total 20$Server::GhostLimit 65536
Sorry for taking so long to repsond, here's the gamemode.txt:Bolded stuff is the things I have added.gamemode.txt is the only thing I have edited from copying and pasting Speedkart.