I'm trying to get a Java 1.1 applet game working. I see other people do it, is it just by guessing the frame rate delay?
No, they don't guess. They calculate how much clock ticks drawing one frame should take, if it took less, sleep for the remaining amount of clock ticks. How accurate this is depends on the resolution of the used timer.
Even if it was 1.4, I don't think I can use GageTimer at all with applets. I only know about 1.5 applets being able to use the proper nano timer. Is that true?
gageTimer works on from 1.1 up to 1.5, it delivers a higher precision on certain platforms. Like nanotimer on 1.5. You won't need to bother with this as you can use the generic loop i described above and let gagetimer handle the timer implementation.
You could extend gagetimer a bit with the alternative timers i described above to support more hirestimers available.