Borrowing from Dean Whitehead:

Research Gate and Google Scholar use different metrics to include and recognise citations. The same is so if you use other related databases i.e. Scopus and ISI. Google Scholar tends to be the more generous in terms of the range of sources it includes in its citation i.e. theses, reports etc. That is why the numbers differ. There is no current way of linking the profiles.

This is a frequently asked question. See., e.g., these related discussions (the list is not complete):

https://www.researchgate.net/post/In_Google_Scholar_my_master_thesis_got_a_citing_but_in_Researchgate_that_does_not_visible_Why_does_it_happen/1

https://www.researchgate.net/post/Can_someone_help_me_understand_why_there_a_difference_in_the_number_of_citations_shown_in_a_persons_Researchgate_and_Google_Scholar_account/1

https://www.researchgate.net/post/Why_is_citation_count_of_research_articles_different_in_Research_gate_from_that_of_google_scholar/1

https://www.researchgate.net/post/Why-does-my-article-citations-appear-in-google-scholar-but-not-included-in-research-gate/1

https://www.researchgate.net/post/Why-some-citations-are-not-recorded-which-are-found-in-google-scholar/1

https://www.researchgate.net/post/Which_is_more_accurate_in_reflecting_research_outputs_Google_Scholar_or_ResearchGate2/1

https://www.researchgate.net/post/Does_anybody_know_how_we_can_add_the_missing_citations_in_Research_Gate_as_will_as_Google_Scholar_profile/1

https://www.researchgate.net/post/Does_anyone_know_how_long_it_takes_for_Google_Scholar_to_pick_up_new_citations_from_ResearchGate/1

https://www.researchgate.net/post/Recent_publications_and_Citations_on_ResearchGate_are_not_getting_indexed_in_Google_Scholar/1

https://www.researchgate.net/post/Is_there_any_method_to_add_missing_citation_of_your_work_in_RG_As_most_of_the_papers_have_more_citations_as_per_google_scholer_but_very_low_in_RG/1

https://www.researchgate.net/post/How_to_include_all_citations_on_RG_and_Google_scholar_citing_your_manuscript/1

https://www.researchgate.net/post/How_can_we_add_citations_in_google_scholar_and_Research_Gate_for_our_articles/1

https://www.researchgate.net/post/How_can_I_link_my_articles_on_research_gate_to_google_scholar/1

https://www.researchgate.net/post/How_to_link_Research_Gate_article_citations_to_Google_Scholar_and_vice-versa/1

https://www.researchgate.net/post/I_have_4_citations_in_Google_scholar_can_these_citations_be_captured_by_RG/1

https://www.researchgate.net/post/How_to_add_missing_citations_on_Google_scholar_Research_gate_and_Scopus/1

https://www.researchgate.net/post/Does_any_know_how_can_I_add_missing_citations_to_google_scholar_and_resaerch_gate