How safe are commercial airliners from gallium infiltration?
Gallium does not interact strongly with steel so it is unlikely to damage hydraulic pipes (these are made from steel, not aluminium). Same source says it also does not interact much with polymers so unlikely to damage the insulated wiring or rubber hoses or epoxy floor panels. It may damage some sensitive point where important device is attached to the aluminium frame, but hopefully most of such places are not single points of failure.
All it can do is to make a hole in the fuselage under the very rare circumstances. Then the aircraft will be depressurized, descend and land. It is probably still not a good idea to allow gallium in the passenger compartment.