See this help page for instructions how to add published or unpublished research to your ResearchGate profile: https://help.researchgate.net/hc/en-us/articles/14293005132305-How-to-add-research. You may upload a book review unpublished research. This will not be regarded as a publication in the narrow sense (see "Is ResearchGate a publisher?" in https://help.researchgate.net/hc/en-us/articles/14292596164753), but readers will have access. There is no special category for book reviews in ResearchGate, neither for published nore for unpublished ones. You may, e.g., select "Data" when adding the review as unpublished research.

writing a book review is a three-step process:

1) carefully taking notes as you read the text;

2) developing an argument about the value of the work under consideration; and,

3) clearly articulating that argument as you write an organized and well-supported assessment of the work.

I post them as articles, but clarify they are book reviews in the description. People do read them, apparently.