The google hits I get on Nomad GT has it down as a 7-speed, freewheel bicycle - not a cassette. The problem you are describing fits nicely with a sticking freewheel mechanism. Do a search for "freewheel servicing", or "freewheel sticking, or maybe "freewheel flushing" and you should come up with a few pointers on how to rinse the mechanism out w/o disassembling anything.
Assuming it's a freewheel, a chain whip won't be of any use at all to you.