I have discovered how to set the background to black (the remaining screen area not used by the Palm app)

add the setting:
-L 0x0000,0x0000,0x0000

(That's uppercase L and lowecase x's)

So a full command with black background would be:

/usr/bin/gvm/gvm -a [app name] -z 1.5 -L 0x0000,0x0000,0x0000

As mentioned before in this thread the app name can be found from FileZ, View and Edit files, Internal, Select the app (to get a check by it), Details, Creator.

Hope this helps someone.