RG and Google Scholar use different data and different algorthms to identify citations. Rather often Google Scholar is also not complete with respect to citations. See also these releated discussions:

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