This happened on a transcontinental domestic flight last month. The gate agent had already announced that the flight was full and asked for volunteers to gate-check their bags to their final destination free of charge. I didn't volunteer. I was standing in the jetbridge waiting to board, when a flight attendant announced that the bins were full and all remaining bags would have to be checked. She then proceeded to grab my bag's handle and rolled it away to check it in. My bag was several feet away before I realized what was happening.

I understand that the bins were full, but shouldn't she have asked first? I had some things in my bag, like my camera, that I would have removed and kept in my pocket if she had given me the opportunity to do so. In this case everything made it through unharmed, but what if it hadn't? I doubt they would accept responsibility. Any thoughts?

If the bag handle wasn't being held by you, a logical person would assume you let go of it to be gate checked after the announcement.
If she ripped it outta your hand, its definitely as issue......
I don't see the issue if the story is as posted.

So she asks, you say no, and she can't magic up space automatically. No point in doing that, it will just lead to unpleasantness.

By making the announcement, she has just told you what is going to be happening - your bag will be gate checked - you should have prepared your bag when you heard the announcement.

Boy Scout motto: be prepared.
I would hold onto my bag until I had removed what I needed from it and then handed it over. Then again I usually am prepared for such circumstances: since I take flights where overhead bins may not necessarily be as big as my carry-ons; and because I know that I don't always make use of my priority boarding privileges and the bins could be full before I board.

Assuming that they weren't pressed for time, they should have at least given people a few minutes to remove necessities (eg, prescription meds, baby food, etc.) and/or expensive items from the bags.