I am a little confused because G4P.setGlobalAlpha() did not exist prior to G4P V3 (i.e. when it was called guicomponents)

Alpha handling changed completely in V3 so there will be some work in converting a pre-V3 sketch. I have looked at the G4P source code and can see no reason why it should not work anywhere inside the sketch code. One thing to be aware of and that is if the alpha of a control falls below the constant G4P.ALPHA_BLOCK (128) then the control is disabled until it reaches this threshold again.

It you are using multiple windows then you can use G4P,setWindowAlpha() to change the alpha value for controls in a single window. Have a look at the G4P_Showcase example that comes with the library (Select Files | Examples from the IDE and expand the Contributed Liibraries | G4P links)

If after looking at the example you can't fix your program then try and create a small example that demonstrates the problem and post it on this forum.