i used 2x LR44 (or SR44) battery for the GX. the 2 batteries occupy one of the battery slot and I use a metal bolt to occupy (short, complete the circuit,etc) the other battery slot. In a way, satisfying the 6V requirement. I feel its cheap and easy solution. Do take note of the material you use to fill up the gap (try to use something with very low electrical resistance). Certain metal will heat up due to resistive heating.
Hope this helps.