Having a problem with my wives 2.0 Malibu

Ed knows everything, just send him a PM

What problems

Cruising at 75mph ac on it’ll run for 2hrs straight no problem.

Once you hit a long hill and it starts using some fuel it pins the temp gauge and high heat warning pops up on dash.
Coolants topped off and other than this it runs well.

Are the fans working? First thing I would check.

Make sure the front air deflector is still there and hasn't been ripped off by a parking lot cement block.

Fans work and deflecter hasn’t fallen off.
Longtime mechanic friend indicated that there known for head gasket problems.