I don't have the FS98 gauge SDK anymore. Am pretty sure you'd need to stick with FS98 style gauges for CFS1...I don't think FS2000 style will work, but it's been so long I'm not sure anymore.
In the FS2000 SDK, the gauge token you'd need to watch will be one of these:
ELEVATOR_TRIM
AILERON_TRIM
RUDDER_TRIM
They return a value in radians, so if you want a different number displayed you'd need to do some conversion math too.
Attached is a simple string display gauge source file that I use for testing variables. It's FS2000 format so it would need some tweaking to work as a FS98 project, but the callback and string stuff shouldn't have changed that much.
You'd have to supply your own header and resource file. I forget exactly how they're structured for FS98.