A friend of myne asked me how to run steam in Admin mode and I had no Idea as I don’t use Steam since I don’t game. He said he needs it to run this anti cheat for GTA any ideas?
There is no “Admin mode” for Steam on linux.
If I’m not mistaken, except the the first two titles, the GTA games are available to purchase via Steam.
If your friend has bought GTA via Rockstar directly, instead of using Steam, there is the feasibility to add it to Steam as a non-steam title.
Anyway, I`m not certain if anti cheat is strictly required for single player sessions of GTA. Should be only required for GTA online which is exclusive to GTA V and would require the battleeye runtime to be installed, as documented via protondb.
The runtime itself has to be download seperately, check steamdb for the link to the store item.
All of them can be run with Steam. Just add the game as non-Steam game, if it’s not a Steam version.
I remember GTA 4 having some really crappy DRM which required jumping through some hoops just to get running even on Windows. Most people opted to download a pirated version, because it was easier to run. 
Ok this seems more in line with what he was talking about. Maybe that is the one he needs. Thanks I’ll have him follow the link you gave.