It seems that you're using an outdated browser. Some things may not work as they should (or don't work at all).
We suggest you upgrade newer and better browser like: Chrome, Firefox, Internet Explorer or Opera

How do I access the option to install the Revision Mod on gog galaxy 2.0?

Under the new extras tab, I do see offline backup installers, both are 1.113fm (revision 1.4.02) nGlide Fix, one being 505 MB and the other being 3.1 GM.

Do I have to manually download and install the 3.1 GB one?
It seems like I was able to install the revision though that extra tab. But now I don't know how to launch it through GoG Galaxy 2.0.

Anyone know how to?
For now it seems like you have to manually start Deus Ex Revision by launching the Revision.exe in the following folder:

Deus Ex GOTY\Revision\System
If the game is installed for the first time Revision is added by default and runs by default if I am not wrong, It is installed as a DLC which you can switch on/off from Galaxy 2.0 under config/installation. I think the DLC addons method will be changed in the future, now it is a bit hard to find.