In ResearchGate, go to the page of your publication, log out (strange enough, it does not work when you are logged in), return back to that page, and use the function "Download citation" with the BibTeX option. However, the exported data seem to be incomplete, at least for books.
ORCID (https://orcid.org) allows to download a list of your publications in BibTeX format. If you do not have an ORCID ID, you may create one together with an ORCID account. It is possible to import the bibliographic data of your publications from several other platforms (but not from ResearchGate) or to create entries "by hand". Then export the list in BibTeX format.
By chance, I found a possibility to use this export function when being logged in: Open one of your publications pages (e.g., https://www.researchgate.net/publication/332727350 - the title in the URL can be omitted) and add /citation/download to the URL, e.g. https://www.researchgate.net/publication/332727350/citation/download .