There are no codes for that yet. However you can try to play with "skipdraw range" option a bit. I was able to remove HUD in burnout games when the option was set to proper value. I suppose the same can be done in GT games.
I found these. L3 turns the hud off and R3 turns it back on.
FrameBuffer
patch=1,EE,D064C794,extended,0000FFFD // Fb on L3
patch=1,EE,204A0C9C,extended,340294D2
patch=1,EE,D064C794,extended,0000FFFB // Fb off R3
patch=1,EE,204A0C9C,extended,DD020068
patch=1,EE,E0040000,extended,00A461F0
patch=1,EE,D064C794,extended,0000FFFD
patch=1,EE,204A0C8C,extended,34030002 // Fb on
patch=1,EE,D064C794,extended,0000FFFB
patch=1,EE,204A0C8C,extended,DD030060 // Fb off
patch=1,EE,E0040001,extended,00A461F0 // Fb on L3
patch=1,EE,D064C794,extended,0000FFFD
patch=1,EE,204AB494,extended,30420001
patch=1,EE,E0020002,extended,00A461F0
patch=1,EE,D064C794,extended,0000FFFD // Fb on L3
patch=1,EE,204A0C9C,extended,340212D8