Its been happening for awhile now, and IMO its a good proactive move by the GAs. If the flight is very full and its likely that BP3 or 4s are going to have to gate check why not get it done at the gate rather than delaying the flight while they try to find space and then get the bags back off the plane to be checked. Also if someone has a 4 and a big bag this gives them the chance to take anything they might need with them out while in the gate area rather than in the plane. In every instance I've seen this happening the GA announced there would be no charge for the gate checked bags.