Hi, Maura-
I wish I had some specific tricks up my sleeve for your mom...All I know is, it takes practice, practice, practice. If she can find a time to sit in front of a mirror and practice different ways of inserting it (usually they recommend putting the lumpy part, the part that goes in the surgical defect, in first, then clipping the clips to the teeth on the other side)... It shouldn't be a matter of shoving it in -- she should be able to be gentle and deliberate and to take her time. Does she have anxiety about the whole thing (which would be understandable)? That may contribute to her getting a gag reflex. I don't know how you feel about meds, but if anxiety is an issue, some anti-anxiety meds might help her relax more while she practices putting it in.
Not a fun thing, the obturator. I have just returned this minute from getting my definitive one, and am feeling a bit shake (I always do after these appointments!). But it seems to be fitting okay. I haven't yet taken it out or put it in myself, so we'll have to see how that goes tonight.
I do hope your mom finds a way to insert her obturator without discomfort -- she needs it to eat and talk, and those are basic necessities for us all! I wish her the best, and will have her in my thoughts. She's in good company -- there are now a few of us on here who can identify with her difficulties.
Best of luck. Hang in there.