Yes, you can walk airside from A to G, but it's a rather lengthy walk all the way through B, C, D, E, and F to get there. If you're going straight from G to A, then I think it might be simpler and faster to walk landside through the International terminal departures hall to get between A and G – especially if you have TSA Precheck.

If you are booked in Copa business or are Star Gold, you do indeed have access to the AC Maple Leaf Lounge. It's in Terminal 2 D Gates. The AC MLL closes at 19h15, so it's of no use to anyone on the usual 01h30 Copa departure, but it just might work for you on the 19h59 flight. Copa starts boarding an hour before departure anyway.

Are you arriving on a UA flight? If so, you'll be arriving in D, E, F, or G. If I were you, I'd probably walk to the AC MLL (anywhere from a 2-10 minute walk), enjoy your time there, and then take the 15-20 minute walk to the Copa gate in A. Since you have the time, you could also visit the United Clubs if you're bored and want to compare. I think the food spread in the UA Clubs is actually a little bit better than the AC lounge, but everything else about the AC lounge is better (less crowded, more seating, outdoor deck, better bar service though I have not personally verified that, and showers.) Also note that Copa passengers do not get access to the UA Polaris lounge.