Is it common for a publisher to offer monetary compensation for a book proposal review?
I was asked some time ago to review a book proposal by a leading academic publisher, and offered to choose between receiving some token monetary compensation or having them send me a few of their textbooks of my choice (I opted for the latter option and remember feeling quite happy when the "free" textbooks arrived :-) ). So yes, publishers offering payment seems not uncommon.

I do agree that the use of a personal email is suspicious and not consistent with professional email standards associated with large and respected organizations like Elsevier. It could be completely innocuous, maybe due to an employee with slightly rebellious tendencies who refuses to kowtow to his corporate culture and insists on using a gmail account, or it could be a scam, who knows. Caveat emptor - obviously do your due diligence before compromising any bank account information. (And let us know what you find!)