I bought a Honeywell programmable thermostat RTH230B can I use it with gas heat only system?

Yes. Hook red to rh, white to w, and you should be good to go.

Now, there will be ways to set up your new thermostat for heat only, heat ac, hot water heat, or, 80 or 90 percent efficeincys. And probably more settings. You can read in the instruction manual that came with the thermostat how to do this.

Let me know if you need help setting this up. (if you have the instruction manual)