Can't find anything on my goodman gdt090-4a online. My house is freezing and the furnace can't seem to keep up. I am pretty smart but I can't even find the filter on this thing. The vents are not blowing out hard and my furnace room is a sauna. Please help.

Filter can be on the side if the air handler, in side the air handler case, above the motor or you may have a central filter somewhere in the house. Sounds like a very clogged filter or constricted supply or return vent lines. We just had to replace some supply lines in a crawl space since raccoons had clawed them apart to make a nest.