When you select A-G mode, it figures you want to attack A-G. It therefore selects your only A-G weapon - the gun in STRF mode.
The computer/radar is too busy computing bullet trajectories to scan, process and display GM/GMT tracks - strafing is done visually.
For another example, take up some slicks and switch to CCIP mode - you lose the sweep there too for the exact same reason - it's computing impact points.
If you want a ground paint without being in a CCRP delivery mode, go back to NAV (or A-A, but I wouldn't recommend it) master mode (take it out of A-G) and manually select GM/GMT radar by calling up the FCR page on an MFD, then hit OSB1 (left button on the top row) to bring up the menu of radar modes, and select Gm/GMT/SEA as desired.
Teej