Thinking of buying a power inverter to install on a vehicle to use for back up power to run my GMP 100 furnace during power outages. Will a modified sine wave inverter work OK and not damage the circuitry or do I need a pure sine wave? Also how many watts does the furnace draw?

Seems to me a GMP-100 is made by Goodman, but I cannot find a GMP-100.

May be best to contact them to ask your question, to avoid damaging any components in your unit.

Air Conditioners, (A/C), Packaged Heat Pumps and Central Air Conditioning (AC) Units for Home Healing and Cooling Systems. Residential and Commercial Heating and Cooling Equipment by Goodman Manufacturing available in 14 SEER and 13 SEER ratings for (http://www.goodmanmfg.com/Home/AboutGoodman/ContactUs/tabid/156/Default.aspx)

I will copy this over at the HVAC forum to see is someone there is familiar with this unit.

Looks like a standard furnace to me.

http://www.hvacpartsshop.com/Gmph075-4%20Series.Pdf