No, it would not be, as the GPL would eliminate the core sentence of their current license:

> You may, free of charge, download and use the SDK to develop a modified Valve game running on the Source engine. You may distribute your modified Valve game in source and object code form, but only for free.

This would be prohibited by the GPL, as it eliminates fundamental freedoms (the right to sell the product).

This sentence is (obviously) very important to Valves ecosystem.