May I upload the preprint version of my article to ResearchGate before publication?
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.