The GPL can't force Valve to release source even if they had used GPL'ed code for an external release. However, a court could force Valve to stop distributing

The GPL can't force Valve to release source even if they had used GPL'ed code for an external release. However, a court could force Valve to stop distributing the infringing binary, and to pay damages to the owners of the GPL'ed code. It is possible that Valve would choose to release source for the steam client after negotiation with the owners of the CPL'ed code instead, but that would require agreement from both parties. And it is actually not totally unlikely that Valve would seek such a solution, the steam client represents a very little part of the total value of steam, the main value is in the content of their rights database, and in their reputation and connection with gamers and publishers.