Needless to say I'm pretty p!ssed off with Jet2. I selected online check-in as I'm travelling with hand luggage only and with a departure time of 07:30 I planned to arrive at GLA at around 06:30 and go straight to the departure gate
I would suggest that if you arrive at the airport at 06:30 you'll still be fine. Check in closes about 40 mins before departure, if there's a queue you will get pulled out and moved to the front of the queue.
For an explanation, airlines have to keep a certain percentage of seats unreserved for those that don't pre-book them, the way to prevent this from happening to you again you should check-in online sooner.