The Gordon Headcrab costume is a crossover costume from the Half-Life series. It is exclusive to the Steam version of the game, and is no longer available due to being a preorder bonus.
Notably, while the upper part of the costume is the special Gordon Headcrab variant, the lower part of the costume is named Gordon Freeman, suggesting the costume shares the same lower part as a headcrab-less variant.
As of Season 2 (Legacy), the beard part of the costume was removed.
As of Season 6 (Legacy), it is possible for a PlayStation player to 'acquire' the costume by linking their Steam and PlayStation accounts to the same Epic account. This of course requires the player to already own it on Steam.