"We were planning takeaway pizza one night."
ShakingAndShocked "Pizza Express and Prezzo both do GF pizza inc. for take out."
Excellent. It really seems a lot easier than it was a few years ago.
OP, Just email her the name of the pizza place and ask her to check out if they have what she needs or if she will bring something along.
Broken11Girl rather a rude response! It's not the OP's fault anyone has food intolerances. This is not about you personally or anyone who is intolerant. The OP has been told by the bride this guest is 'off and on.' I don't think people think this applies to most who say intollerant.
GreatFuckability good advice "... you need to stop stressing about this!"
Comingptrumps It's fine to ignore us but pages of people have said get her to speak to the restaurant direct. And now you have "I've phoned the restaurant, and I'm waiting to hear back from them about whether they've got a gluten-free and dairy-free menu. I'll let you know as soon as I hear back from them."
You do not need to do this. She can speak to them direct.
Re "On X evening.... Do let me know how you would like to proceed."
I'd just say
"As mentioned by email a while ago, on X evening the plan is to order pizza to the hotel. As they do not do gluten free, I'm assuming that you will bring something with you that you can eat."
"I'm also planning something.... I hope this is understandable."
Just say
"I've ordered a cake (it's top secret, if it is) and, unfortunately, as I did not realise you were gluten intolerant the cake isn't gluten-free. I hope this is understandable. You can speak to the restaurant about what gluten and dairy-free desserts they serve."
I must go to bed now. I hope this is all going to work out and as I have said a lot, will soon be over. Please now stop over thinking this.