New to AC forum.

I don't typically check a bag when I fly, so unfortunately when I do, something inevitably goes wrong.

Tomorrow I'm flying to YYJ via YVR from the US on United connecting to the code share puddle jumper. I won't have a problem securing overhead space on my first flight, but I know my bag won't fit on the plane YVR to YYJ. I'm wondering if I can gate check it? If I do, does AC charge for this?

Thanks.