However, in my garage I have a MWBC situation, which seems to pose a problem.

You see, in the garage, an MWBC wire carrying two phases (Black+Red+White+Ground) comes down from the attic (I assume) and feeds a 2 gang receptacle with each one of the two receptacles being wired to a different phase. However the problem seems to be that this MWBC wire then CONTINUES (inside the wall) to feed additional 2 gang situations on other walls of my garage - and all these downstream 2 gangs are wired to the MWBC wire exactly the same (that is one separate phase feeds each receptacle, the white neutral is shared)

So seems like it is NOT POSSIBLE to protect this circuit with CGFI because if I replaced the first 2 gang in the chain with CGFIs the white wire would be common downstream and I imagine that would cause the CGFIs to trip on unbalanced current whenever anything was plugged in the downstream 2 gangs (I have not tried it but reading how CGFIs operate I suspected there may be a problem).

Assuming I’m correct about being unable to use CGFIs in this situation, is there something else I can do to get CGFI protection on this circuit? Or some other method offering similar protection? This is the garage, so I imagine it may be even more important to have some protection.

I would like the CGFI to protect not only the outlets but the wiring too for the following reasons:

the MWBC wire goes through some exposed metal conduit and metal boxes in the garage (some of the wire is in the wall but some is exposed conduit that I can tell). I do not know how well my house is grounded, which is one of the reasons I wanted the CGFI in the first place. So if I just install a number of CGFIs (I would need 6) the conduit and other metal work would not be CGFI protected.

I also just found out that this MWBC wire feeds the two outside outlets in my back yard (seems like there was an old Jacuzzi area there which previous owners took with them). Apparently the MWBC travels to the outdoor outlets through PVC conduit. I do not really need those outdoor outlets so I could disconnect them really but if there is a way to preserve them I would rather keep them.

As pointed out, you can't use 110V GFCI devices to protect MWBC. You'll need a 220 (with a load neutral) breaker to protect the whole circuit if you want to do that. Otherwise, as previously suggested, use individual 120 devices. You can not share the neutral on the load side of a GFCI with a different (including the other half of a MWBC) circuit.

If the GFCI can protect against an improperly grounded exposed metal conduit, or metal box, or the outdoor cable coming in contact with moisture in wet weather, why not have the GFCI not only protect but also act as an early warning that something is leaking somewhere down the line (bugs making nests in the outdoor outlets and conducting in wet weather, mice having chewed on the underground cable etc.)?

While a GFCI might help you detect mice chewing on wires, more importantly it might save someone from being badly injured or killed from an electrical shock (say when they cut through their extension cord with the hedge clippers).