It's referencing a coping mechanism when you lose in team-based multiplayer games to blame other team-mates instead of looking inward and accepting you might be the problem.
"GG" is short for good game, it's basically the polite online equivalent to a handshake after a sports match, or any competition really.
"No heals" is the blaming of ones team for not helping out, in this case healing which is often done by a designated healer class which a player can pick in the game. Purpose of this class is replenishing health points to other fellow team members, something which is needed to give the team a better chance of winning as the opposing team is trying to kill you.
So logically, if the healing is either bad or lacking, one could surmise that "no heals" is a valid criticism, but "it takes two to tango" as the saying goes, and if you run away from the healer, out of their range or too far into enemy territory that the healer can't help you without dying themself, then the problem isn't the healer - it's you.