If you can run FSW (Full-screen window) mode, IE: have dedicated menu up on top of your room. Run closed online session ? Why ? For one thing you have "Car Select" ( example you can run every GT car in a session and compare times without reloading) Easier to boot /add cars IE: Add Ai in dedicated menu as fast as you can click a mouse ..........rather then offline , add 5 wait, add 5 wait, add 5 wait ....... equally when you boot offline is 2 mouse clicks, dedicated menu is click click click gone. Change some session and room settings with just a reload track ( faster then exiting, changing in menu and reloading )
This is good advice for single player games. Also the major advantage of loading an .ini file with individual mass for all the AI drivers, so you can get the same teams and drivers consistently out front from race to race. Saves a lot of editing of files.
Check in the session settings that "Private Practice" is turned off. Then set number of AI in the session settings or after loading the track click on "Standings" and then "Add Car" at the bottom.