04 gto wont heat up
 

In the city it'll go up a few ticks and the heater will work decently (it's roughly 30F outside). Once on the highway it'll just drop all the way back down to nothing on the gauge and the heater will blow cold. I just replaced it with a gates t-stat and housing. Even though once i cracked it open the old stat was closed..the o-ring on the part that opens is a little tore up. Boiled it and it opened at around 188F. Haven't driven it yet with this new stat but judging by the old one nothing should change. What should I look into next? really lost at what to look at... unless they can not work and then work.

Previous owner put a bigger rad and new water pump in it the stat and housing looked factory (85k miles).

Dumb ? when putting the new stat and housing on i had to push it a big to get the bolts to go into the waterpump. There was a little under a 1/4" gap between the housing and waterpump. Can't remember if that's normal or not.

I had a failed thermostat and it behaved exactly as you describe. Some of the thermostats are a failsafe design that defaults to open when they fail. That means low temperatures on the highway and little heater output. You may even get a code about failing to reach operating temperature. I have an 04 GTO with the newer-style thermostat. There wasn't a need to pull to down with the bolts. I don't know about the older style - never had one of those.

Bad heater core?

I've got a bigger radiator in mine, with the factory t'stat and it still seems to run on the warm side....too me. It still hits 201 in traffic. Even got the fans set to come on earlier. The gto has a vacuum operated valve inline on the heater core lines. Supposed to keep water from flowing thru core, if the heater is not on heat. Maybe it's not opening. Also...I have the higher flowing water pump. When I was ordering it...they was 2 types. The standard flow direction and a reversed flow one...looked the same. Could the previous owner put the wrong one on?