I wondered whether or not the selection of articles and documents proposed by ResearchGate was based on Boolean logic and with, more or less, the same logical operators as Google
If you mean the search at https://www.researchgate.net/search.Search.html?query=&type=publication: The help page (https://explore.researchgate.net/display/support/Discovering+and+requesting+research) does not give any details. It seems to me that it is a very simple search connecting search words with the logical AND operator. There seem to be no sophisticated search features.
To me it seems that at the moment not even an AND-search works (no matter whether "AND" is actually used, or not). Some weeks ago, I searched for and got 79 results. When I do it again now, I get 100 results pages, i.e. 1,000 results, but 90% of the results do not contain all three search words. Does anyone know, whether ResearchGate has changed its search function and how one can actually perform and AND-search?