Rick,
It sounds like you have a wiring problem on this circuit. The GFCI (Ground Fault Circuit Interrupter) detects the presence of current on the ground line (bare copper) or a short between the common (white wire) and ground. The XPS3 switches does not connect to the ground wire not connect to the ground wire - is should not be the source of the problem unless something is mis-wired or you have a ground wire shorting in you box.
XPS3 wiring:
http://www.x10pro.com/pro/pdf/xps3.pdfI have many X10 devices on GFCI breakers in my basement and garage (I'll admit that none of them are XPS3's) and have no problems.
There is a thread on this subject here:
http://www.x10community.com/forums/index.php?topic=11038.0And a nice description of the GFCI operation here:
http://www.codecheck.com/gfci_principal.htmCome to think of it, why are you installing a GFCI on a line with a relay switch? Do the switches control an outlet or a hardwired light. Switches controlling hardwired lights are not normally protected by GFCI's (unless I'm out of date on my wiring code - possible).
Are you using a panel mounted GFCI breaker, or a GFCI receptacle (outlet)?
Sorry for all the questions - I'm trying to get a handle on your configuration.