^^Would be easier to create a new face from scratch, also it would kill the joke if he doesn't look like Gordon.
Just imagine all the players that are used to get the door decoder from Dr. Doak, but this time they'd face Gordon Freeman.
Even if we can find a more suitable Dr. Doak (e.g. Major), i'd still like to have Gordon as an easter egg somewhere, maybe as a regular scientist.
Since there isn't even a model of him in the game itself, we'd either have to do one or ask one of the creators of the Freeman GMOD models for permission to use it.