Am I allowed to my article to researchGate which is so far only published at arXiv? I understand that in general, it depends on the publishing license of the journal if I am allowed to distribute my article further. However, I want to know if I can upload an article, which I submitted to arXiv prior to publication in any journal (which is allowed by the journal I will send it to in the end), to researchGate?
arXiv uses a default license that does not transfer copyright but rather only grants arXiv a perpetual, non-exclusive license to redistribute the work, similar to, but less restrictive than commonly used Creative Commons Licenses, which arXiv also accepts as alternative licensing for submission.  That means you retain the copyright on any articles you submit, while providing a perpetual license to arXiv to do redistribute it.  

Since you still hold the copyright on your article, you may do anything you wish with it, including submit it to researchGate.

The issue you need to be concerned about is whether the journal you wish to eventually submit the article to considers submissions to arXiv and/or researchGate to be "prior publication".  Some journals only accept articles that have "never previously been published" and different journals define what constitutes "published" in different ways.  Some journals exclude arXiv, researchGate and others (along with conference presentations, etc.).  Other journals will not accept papers that were previously on those (or other) archives.  Check the policies of the journal in which you wish to eventually publish the paper to ensure that you will still be able to after submitting it to researchGate.