The GPL is often misunderstood. There is the saying "free as in speech, not as in beer". Free speech means that nobody can stop you from saying what you think, except you can’t say that you are going to blow up the President, or shout FIRE in a crowded building, so there are certain usually rather obvious restrictions. Free beer means you don’t have to pay for it.
The GPL is free as in speech. You can do what you want with it, with certain fairly reasonable restrictions. For example, if the link at the bottom is hard-coded into the script, you must keep it. If it isn’t, for example in MODx it’s only in the default template, and if your template isn’t a copy of theirs, then you don’t have to have it. But in the Manager login form, it’s hard-coded into the script, so you must retain it, even if you modify the script (which I have done, see
http://www.sottwell.com/manager).