There is a distinction between the operator and the maintainer ratings. Also a distinction between mandatory equipped and optionally equipped. Also a distinction between US waters only vs, operating internationally. A significant group of GMDSS students are in the optionally equipped fishing fleets and recreational sailors; probably far outnumbering merchant marine; with equipment that does not include the satellite equipment or backup power.
The question banks are available free online as a download.
As stated, those who take the 15 day CG GMDSS course; required for some deck officers, don’t need to take the FCC exam to be an operator. I believe few engineers go this route. The only route to a maintainer I believe requires the FCC process of GROL and maintainer, and usually radar endorsement as well.
If you are going to have both deck and engine licenses operating internationally on a mandatory equipped vessel; I believe you need to do both, but FCC will give you part 7 credit.
If you are a mandatory equipped vessel on a voyage of 7+ days I believe someone has to be a qualified maintainer, and its usually an engineer or specific electronics/comms officer
W5YI materials I believe are still the go to study method, about $80 new, and commonly passed around, I got mine as a hand-me-down, but there are several free websites with pop-up question pools and etc. These sites are horribly dated and poorly formatted, so if you provide a good website you will get users.
1-1 semesters of the most basic EE or EETech courses makes actually knowing and learning the materials much easier. If you are actually going to repair and maintain, it helps if you did more than memorize some multiple choice answers
The old US Navy basic training manual for electronics techs & radiomen is still a solid go to reference and very low cost; and I believe a pdf of it is available free online.
There was at least 1 coaching program to do GMDSS O/M w GROL radar endorsement and amateur extra FCC licenses together with video tutorials and etc; and testing in 1 very long sitting. Appropriate because the most common HF GMDSS radio in the fishing and recreation fleets is the Icom which is also a fully function ham radio with ham email, digital, cruising nets and etc. The ham community is really good at teaching real skills and knowledge beyond memorizing multiple choice answers.
Also, I believe the CG rating is for 5 years and must be revalidated. The FCC license is once for life.