You don't automatically un-use it when you log off?

^^^^^^^
Thats should work thats what i do when i wanna take off a texture pack unless you actualy downloaded it to your computer then you have to go take it off then put your normal one back

No but the GTA texture pack, in this case, is a "download everytime you log on" one, so I restart my minecraft the texture pack is gone, and once I rejoin GTA it downloads it again

Yes it auto downloads since you accepted the 1st time every time you join it will insta download / put the texture pack on