Just go to a second channel strip -> preamp screen, select the same input at upper left and select Trim on Surface on upper right.Leave first channel strip Preamp on Surface. Now you have 2 different channel strips with same gain but on the second channel you have the Trim Gain to adjust as you wish.All other settings are different so different eq/comp etc for monitor and foh. I think it’s the common way to do such thing in GLD as other users commented before.
Once you do it go to MAIN L/R Routing and set OFF the “monitor strip” from the MAIN L/R so you dont accidentally move the fader or if you want to use a second reverb for monitor, post state (so you must move fader as needed but you dont want sound to main l/r)
Proud GLD Owner