Generally the GPL will tell you that you are not permitted to use any component of the software that is provided under a GPL, in a proprietary piece of software. If you use GPL as it's basis, you HAVE to keep it GPL.
What's more depending on what the software is you are tweaking, you may be required to send the changes back to the person who owns the GPL for that item. That way they get to choose all the best tweaks and additions and include them in the next version, and refuse the ones they do not consider good.
This is to reduce the risk of splintering, as happened with Unix. Unix was such a wonderful OS, but everyone went off developing it on their own and they ended up with such vastly different versions that what ran on one version wouldn't run on another. So much so that it becam useless as an OS, and this ultimately was it's downfall.
So play nice! If you change it, pay back the favour that was having the GPL template or piece of software in the first place!