I have got gta v on A USB from a friend, I have put in the steams apps, common folder, but when I click install on steam it says not enough disc space. I have a 118 gb computer and I can only just fit it in but it does fit with 2 gb left. Is there a way to let steam recon use that it is on my computer?
There isn't enough space, it is correct. The initial installation is 118GB, but after DLC, multiplayer caches, saves, etc. etc. the 2GB left over won't be enough. Clear enough 5GB or so off your HDD before you install the game.
You can try to copy gta to another drive, start installing it on steam. Wait till the first bytes are done. Close steam, copy the data back, start steam and you should be happy. The download resumes, load the last bytes and should be done.
This normally works for me.
Btw. This way you will see how much space is needed AFTER installation, due to the fact that you start the installation normally.