After a bit of help from RevEng and reading his post about it here, I was able to add High Score saving to Gate Racer using the AtariVox or SaveKey. Your high score will automatically save and be displayed on the titlescreen. It saves only one high score. I decided to create a new post about it since it's no longer a part of my "2k game experiment". The game needed to be expanded to 4k to fit the extra save game code in. I made a few other minor changes, but the game is pretty much the same as the 2k version right now. Changes were very minor and I'll probably make some more changes in the future. You can now press F2/Reset during the game to go back to the title screen and see your high score.
The save feature works in Stella too but you need to activate the SaveKey on controller port 2. This can be done by launching Stella from the command line with stella -rc savekey game.bin or in the GUI by highlighting the game, clicking on Options | Game Properties | Controller Tab | P1 Controller | SaveKey. If the titlescreen score remains a blank white strip on the bottom of the screen after more than one play, you probably haven't properly activated the SaveKey in Stella. I have not yet tested this on real hardware but it appears stable in Stella. I'll test on real hardware after work this evening.
Note that Richard Hutchinson maintains the memory allocation list for the AtariVox and SaveKey here. The current version of this game will write to the "Scratchpad" area of memory. If you use it on real hardware, it's possible another game that uses the SaveKey could overwrite the data. I need approval from Richard to be permanently assigned an area and I haven't requested that at this point.
You'll still need all of the modified include files (in the attached zip) from the original 2k Gate Racer if you're going to compile it yourself, along with the Avox include files from RevEng's post here.
EDIT: Revision 'i' should be the final version barring any bugs found. Here are the changes:
Fixed savekey initialization issue (thanks again to RevEng)
Added permanent savekey location for real hardware at my assigned location (I used $0640, $0650 and $0660)
Holding the joystick up and pressing the game select switch on the title screen will clear out the current high score
Hitting oil slick now has the same behavior with velocity controls turned on or off
The oil slick now resets to top after a crash
Hitting the oil slick now produces a skidding sound
One other important note about saving: The high score does not save until you press the fire button after a crash.
3/24/13: I noticed a bug in the final version of Gate Racer II, if you hit the side of the road you may get stuck in a loop where the game won't reset because the car sprite's location isn't reset properly. The updated version (b) is attached, it was a simple one line fix.