5 cells will be a 6V pack, and most RC gear can cope with this, is the GWS Rx the indoor micro one? the planes that fly these can have 8 cells no problem, the speedcontrol takes the voltage down for the Rx and servos. 6V should be fine tho. however a fully charged 6V pack can be as much as 8V.
the info that came with your gear should give you the specs and how many cells you can run it on