Garry's Mod Add ons taking up too much space

As much as I love Garry's Mod, the add ons and automatic downloads are taking up way to much space on my computer. I have just over 1 GB left of space on my HD. I was hoping someone would be able to tell me where I can find all of the add ons that are a part of Garry's mod on my computer (because there doesn't seem to be a clear cut space for them any where) so that I can either move them to an external HD or just delete them. I am using a Macbook air (I would get a PC but this is a school provided laptop) I haven't upgraded to OSX Yosemite yet because there hasn't been enough space for it and If i were to move the add ons to the external HD it is a 2 TB seagate backup plus slim, if that helps. Also in the future if i were to get a game on steam would it be possible to download it onto the external HD and play it from there without having to have any of the data actually on my computer?

Thank you for any and all help

Garry's Mod Add ons taking up too much space