I am in Platinum lounge in BUD where I got on my Gold A3 card flying Budapest to Athens on A3. Since we are positioning to ADB for our Qatar J to DPS, we have checked bag and needed to get to counter upon arrival to airport. I will not mention that the agent was rude, arrogant and everything but not perfect, at the end she assigned as our preferred 12 Alfa and Charlie, but things got weird at the end. I understand they must tag your carry on for whatever reason, but we were told we should check my carry on now for free because of the full flight or they will collect 50€ at the gate if I dont board with first 50 people.

I would absolutely understand this behaviour if my carry on was oversized, even I would find it surprising to "punish" the top tier airline member, but what If I did not visit counter and then was surprised at the gate to check my carry on (involuntarily) and to pay 50€. Is not this against rules? Is not this against common sense?

I just denied to check my carry on as I already checked on bag after years of and hundred of flights and simply told her that I board first as gold member. Now I will just stay behind the beep to see what they doing.

One more note, at least agent handling entrance to fast track let my girlfriend to go with me even she is just blue, which was nice.

Bollocks. As long as your carry on is within size and weight limits it is against the terms of carriage to force you pay for it. If the flight is full and they believe there may not be enough overhead space they retain the right to force you check it in, but that should of course be complimentary. No one can have you pay for a service that is clearly stated in T&C to be optional and you didn't opt for (i.e. checked bag).

I usually fly A3 HBO to skip waiting for my bags, especially in ARN... My carry-on is always within the permitted size (and most of the times weight) limits, and in fact I have never even had it measured/weighted save for LHR T2.

Could it just be that the person who told you this isn't an Aegean employee, but rather handles all kinds of airlines at Budapest and is used to passing on this info due to the majority of flights being Ryan and Wizz? :P

PS. Ryan's "argument" for pricing carry-ons actually stands, since a A320 does only have space for ~90 cabin bags, if one makes use of the size allowance to the maximum, I think we've all had the situation on an Aegean flight where we just couldn't find space for our bag upon reaching our row...

Most likely she wasn't working for Aegean but rather for a handler. This makes no sense either way though. If the agent thought that OP had a non-HB fare, then she would have charged him/her no matter the potential storage space availability onboard. Ryanair and the likes are pretty good at leeching ancillary revenue from pax in such cases. Otherwise, since OP's carry-on was within the applicable size and weight restrictions, the airline would have no right to charge OP even if they forced him/her to check that bag. LCC or nor, it's simply against the law to charge somebody for a service they didn't purchase (directly, or indirectly by insisting to travel with an overweight bag).