May I upload the preprint version of my article to ResearchGate before publication? Recently, I got an acceptance of a journal paper, where I am allowed to self-archive its preprint version in my own repository/website. 

My question is whether I am allowed to upload it to RG before publication, assuming that my RG page is a personal web page. If 'yes', what should I mention in the publication date entry, knowing that it is compulsory?
First you should note that ResearchGate is unlikely to be considered self archiving by the publisher. While they may allow it, you should confirm. You should also ask about embargo periods. Assuming the publisher has told you the publication date is compulsory when self archiving, it is not unreasonable to ask what you should do at this stage.

In the absence of guidance, you could use something like forth coming or in press in place of the date.