And having mouth force filled with marshmallow creme, that, I would hope you are NOT doing in such a way that it cuts off a windpipe and chokes the submissive, still means that all of the mouth is filled, no ability to breathe around the marshmallow creme except through nose if passage is clear. Theoretically the marshmallow creme should dissolve or be able to be swallowed.
The pump gag is hard, not soft, not forming to the mouth. The advantage of the pump gag is that the submissive should be able to drool.
In both cases you must be careful the submissive will not choke on their spit.