I have created a LaTeX class file. Can I release a class file created by me under gpl?
This question can't be answered with a simple yes or no (despite the comments above made by one of the most knowledgeable person on this site).
It largely depends on whether or not your class is derived from other work that is based on LPPL or not and most likely a good proportion of a real class would be of that nature (unless you done your class by loading a different class via
The reason is that while both LPPL and GPL are free licenses they are incompatible with each other (and for a good reason). The main goal of LPPL is to provide freedom for change while maintaining a core feature of LaTeX which is information interchange and that requires that if you build a LaTeX document it will work not only for you but also on a different installation (provided the same packages and classes are installed).
LPPL therefore requires that you identify your work if it is derived from some other work as being different from the original in a way easily identifiable for a user of your work, e.g., that a user does not mistake your work for the original.
For example, if you take
Now if you want to distribute your
For the same reason it is unfortunately utter nonsense if somebody claims he or she is dual-licensing some work both under LPPL and GPL, it really invalids both licenses.
A much more elaborate explanation and some information on the history of LPPL can be found in Reflections on the history of LPPL which appeared some time ago in TUGboat. In there I think (or at least I hope) you will find a good argument why GPL --- while it is a very good license for a lot of situations --- is not the best model for works in domains that deal with information interchange, i.e. that need to protect the integrity of a "language" as well as all the other freedoms we wish to have.