I don't get what you mean by saying 'and installed in the usual folder of F12013'
It should be installed in it's default folder. GameSave Manager does NOT backup anything in the install folder anyway. It backs up data in the steam/userdata/xxxxxxx/226580 folder (F1 2014 profile folder)
Ignoring all that, you will just have to wait until GSM supports F1 2014. If you can't wait for the updated game list, then you will have to create your own CUSTOM game save. You will have to read GSM's manual to learn how to do that. (Actually see the tutorial link below)
If you make you own custom backup, then you need to point it at this folder
:-
SteamInstallFolder/userdata/xxxxxxx/226580
xxxxxxx = Your personal Steam ID number
Or just make a backup copy of that folder manually (copy and paste somewhere).
So either back it up manually, make a custom GSM game save, or wait for GSM to add the game to it's supported game list.
Here is the tutorial on how to make you own GSM CUSTOM game save