How to stop mod to download source files?

i just bought garrys mod but i have problem.
somehow it tries to download all the source games like css , hl2 serie , portal and random mods. that happened before, when i selected everything in extensions menu of the game but i fixed it by deleting some config files in game folder and verifying integrity but i cant remember how and it happened again and i didnt do anything to extensions, i tried to delete cfg , lua and some other folders that i thought they might have something to do with saved variables but nothing change ,after clicking the game it freezes at Loading part and never "load"s ,if i alt+tab and look at my hardrives i see that 20-25 gb is written.After closing game and s team via task manager and restarting i see that that games is on queue for downloading and if i delete those and start the game it still tries to download again. So how can i reset that setting by not getting in to game and not re-installing the game and prevent it from happening again?
p.s.1:after some thoughs , i think those settings kept in steam cloud , anyway to reset it?
p.s2.: soz for my English.

Garry’s Mod is a mod, not a standalone game, you need to download the other games via Steam.

Garry’s Mod downloads is 4gb which includes source models, source materials , source sounds, source 2007 materials and garrysmod content. i want to keep it that way and prevent it from downloading more.

Doing that will just result in not having all that extra content.
Also the only way to stop it is when GMod starts is to alt-tab to steam and stop them manually.
Just let them DL that’s what I do. Better yet full install them and have fun with the other props and maps.