Since the code is under the GPL it basically means that you can use and alter the code as you like, as long as whatever you release is also entirely under the GPL. While this does not in any way implicitly prevent commercial distribution, the GPL requirements make normal commercialisation infeasible. You must make the entire source code available, and people are free to alter your code in any way they like, distributing their modifications (or lack thereof) either freely or for a price - profits of which you are not entitiled to. If you do not intend to distribute your modifications (i.e. if you are making a forum for personal use rather than for redistribution) you can use and alter any GPL code without any requirments to make source code available.