Lifestyle tips

You don't ever mention your gf's total gain, but I'm guessing it was 50lbs or more over about ten to twelve months? (Based on writing off the quarantine 25, gaining a few then really taking off). Working remote and being quarantined she probably not only gained fat, but lost muscle mass. I find when I gain a lot quickly it gets hard for my muscles to keep up and comfortably support my day to day activities. Rather than a hike or bike, start with some strength builders like squats and fire hydrants. The goal isn't weight loss so you could even reward with snacks (unless she wants to lose it, which you might want to discuss if she is unhappy and continuing to do this "for you"

I'd suggest starting with activities to do together, rather than anything that sounds like 'exercise for exercise's sake'. For example put on some favorite music and have your own mini-disco in the living room or kitchen. Or throw a ball or frisbee back and forth, or kick a ball between you. Just get the habits back of getting up their feet and moving around and enjoying being physical. If she resists that, then I think you are right to be a bit cross with her, saying "I don't want to spend my whole life sitting on the couch. I love the weight you have gained, but it is possible to be this size and to also get up and go outside and do things. If my suggestions don't sound fun to you, please think of other ideas."

If the goal is cardiovascular health and not to necessarily lose weight, I would suggest 20-30 minute slow walks at her pace. You could even get ice cream or a milkshake after. You won't see the numbers on the scale go down ( they might go up) but the walking should help with heart health, muscle loss, and overall health.