2014 Outback, 140k miles, how likely is it to be the head gasket?

Just noticed today that my Outback coolant temp gauge was creeping up when idling at a stop light. When i started moving it went back to normal. The coolant res was slightly low so I topped it off.

Drove the rest of the way home and saw the temp creeping up again unless I hit the gas enough to get some good acceleration.

In the past few weeks I have also noticed a little bit of hesitation (not sure the best word) when slowly accelerating, but it goes away when coasting or accelerating faster. not sure if this could be related.

I know these symptoms could be anything from a bad thermostat to a head gasket or more, but what are the chances that its going to end up costing me an arm and a leg?