My answer is in no way any sort of legal advice. I am not a lawyer and have actually no idea how copyright and licences may or may not work in your country.
The GNU.org website pretty much covers it all. I quote (emphasises by me):
If a program P is released under the GPL that means any and every part of it can be used under the GPL. If you integrate module Q, and release the combined program P+Q under the GPL, that means any part of P+Q can be used under the GPL. One part of P+Q is Q. So releasing P+Q under the GPL says that Q any part of it can be used under the GPL. Putting it in other words, a user who obtains P+Q under the GPL can delete P, so that just Q remains, still under the GPL.
If the license of module Q permits you to give permission for that, then it is GPL-compatible. Otherwise, it is not GPL-compatible.
If the license for Q says in no uncertain terms that you must do certain things (not compatible with the GPL) when you redistribute Q on its own, then it does not permit you to distribute Q under the GPL. It follows that you can't release P+Q under the GPL either. So you cannot link or combine P with Q.
Further the GNU GPL v2 states that (emphasises by me):
You are not required to claim a copyright on your changes. In most countries, however, that happens automatically by default, so you need to place your changes explicitly in the public domain if you do not want them to be copyrighted.
Whether you claim a copyright on your changes or not, either way you must release the modified version, as a whole, under the GPL. (if you release your modified version at all)
In my country, the GNU GPL in any version is more or less invalid, even the public domain. The copyright laws of my country prohibits me to take, reuse, redistribute, change, sell or in any way alter anything that was not done entirely by myself and remove the original copyright claims (or alter them to display myself as the copyright owner) without a PRIOR WRITTEN PERMISSION by the true copyright owner. These do, however expire for certain things. For example artwork copyright expires after 70 years of the original person's death, even if the copyright was transfered to the authors relatives.
De-facto I may use the GNU GPL as a written permission and do whatever I please, unless the author states that I may not do this or that with the code (then that would have preference before the licence agreement by the GNU GPL).
This may be different in your and the copyright's original authors country though.