Has she sent out any invitations yet? If she has, I think you might be stuck. I’d definitely not just decide not to go, if not for her, then for everyone else who shows up to be there for you and bring you gifts. Although it’s completely insensitive and ridiculous of your future mother in law, the gracious thing is to show up anyway.
If she has not sent invitations, could you talk with her yourself, instead of leaving it up to your maid of honor? I know the bride isn’t really supposed to be troubled with the planning of anything, but in this instance it might be better to step in. She might not completely understand what having a gluten allergy means, and need you to explain to her that although you appreciate that she wants to throw you a shower, you would like to be able to eat along with everyone else. You might suggest some other ideas for affordable restaurants or even find a gluten-free pizza place (they seem to be becoming more common, at least where I live).