May be the reason is that Google Scholar considers self citations and research gate d uncounted self citation in the Research Gate does not. Are there any other reasons?.
I agree with Sajda, this has been discussed a lot on RG. Difference in the number of citations in GS and RG is due the use of different citation tracking system by GS & RG.
Google Scholar tracks academic articles, but it also counts theses, book titles and other documents towards author citation metrics. RG on the other hand tracks citation only if the paper citing the original article is available in their database.