All code in the published GPL application written by me and there is no 3rd party GPL libraries in it.
Do I still keep the whole ownership of the code even after the GPL release?
[1] I know that people got rights to use old code or fork it which I'm fine with. But I don't want to get sued because I stole my own code :)
As far as I can tell, you, being the owner of the code, can do whatever you want with it, including relicense it.
I don't think you'll be able to "take it back" from anyone who still has it, or prevent them from GPL'ing a fork or something.
You can license your code under multiple licenses. License it as both GPL and proprietary (like Qt used to).
Yes. You can release the code you have written yourself any time again under a different license or do whatever you want with it. You still keep the copyright on that code. GPL only applies to "copying, distribution and modification" as stated in the license itself.
But you cannot take back code you once released under GPL , you have to keep it available for the public (see comments).
Can I decide to change the license of my published GPL application to something closed-source
Of course. The key is that it has to be yours and only yours. If you've accepted patches from other people, then the case is not so clear cut. Safe way would be to strip your code of these patches, and change it's license. Although in some legislatures, the contribution must be "significant" to even take in account, so you might get away with changing license of the code with patches.