i think what you have to do is get a score of 500+ on the cold challenge for a chance to unlock gabriel. which will likely mean needing to use pol as he is the character with the highest cold tolerance.
Im using Pol, skills that are supposed to make his cold resistance higher and that helps me make more combos, but it's just impossible to not get a "brainfreeze" and have to lose precious time to reach that score my max score right now is just 245... it's a bit frustrating honestly, I found out that the """"cold resistance"""" is how fast the character can "recover" from losing temperature, shouldn't the cold resistance also allow you to recover from brainfreeze at a faster pace? also, shouldn't cold resistance make you take less "cold damage" from cold food instead of fast "regeneration"??
being able to recover faster from it wouldn't be a bad idea to bring up with the dev. but if you are still getting brainfreeze that often it might be cause of which foods you are choosing. it might seem tempting to go for the higher calorie options. but with beer, ice cream and soda causing so much cold. you may only want to use one of them, maybe two but never three. though i forgot the recent update that limits your combo multiplier to 10 without a special skill that only certain people have made it even harder to get there.