Actually, you just cannot file "/G" for IFR flight plan if you do not have IFR-approved GPS. --- the only avionics requirement for IFR flight is "that equipment suitable for the navigation facilities you will be using" (or something like that). As far as DME requirement, there is not a DME "requirement", UNLESS, the approach you will be using requires DME, or, if you are asked to do a DME defined hold. If you have filed appropriately, your equipment capability would show "/U" to indicate no DME, but transponder with mode C.