98 century 3100 v6

Double check your vacuum lines. A manifold vacuum leak can cause symptoms similar to what you are describing and you would have removed some to do the gasket change.

I've got all the vacuum lines reconnected and none of them seem like they're leaking. My throttle position sensor put up a code, so I'm wondering if it isn't that. If it isn't, how can I get the butterfly valve to open with the throttle?

It operates fine if we manually play with the throttle cable, but at an idle it's running really rough and under load it won't accelerate past 2k rpm

Adjust the cable, maybe?

That's what I figured but it wasn't tampered with enough to need to be rearranged

Check to see if the spring inside the throttle position sensor (TPS) is broken - you will need to take it off the throttle body to do so. It will be obvious when you move the "D" shaft on the back plate of the sensor, and it provides no resistance. If the spring is broken, the TPS will not correctly note the position of the throttle plate, and subsequently send an incorrect signal to the cars computer. This will cause issues including those you have noted.

you didn't do the gasket correctly and you still have a LARGE vacuum leak. Go back, re do the work but this time, take your time in doing so. that intake gasket can be tricky. check your torque specs. the 4 middle bolts have a different torque spec then the outer, and the outer bolts have 2 that are under the manifold. make sure you have those tight, re do the work.