As for me, I don't know any other solutions.
Can you tell me more about China GPS and JTAG?
Long time ago I was trying to play with debug registers of 37xx, in particular, I was trying to set a hardware breakpoint.
Unfortunately, I couldn't enable debug registers, though I was doing everything right. I think it's because debugging capabilities were disabled by manipulating specific output pins of the ARM core (I'm not sure if those pins really come out of the processor). And, in fact, there were a few flags in some status register (I don't remember its name) indicating that.
So, in conclusion, I think Garmin disables debugging support (including JTAG) for production processors.