cats refuse 'gastro issue food' [been to the vet already]

TL;DR Cats wont eat the gastro food, but they will eat chicken is that ok?

I'm trying to keep this short because it is a very long story, and have tried to keep the key information intact. Sorry it sounds a bit disjointed, I also don't want to name the vet for reasons.

We (mid forties couple) had to bring 1 of our 3 neutered cats to the emergency vets (6 yr old male). A day later our 2 other cats (6 yr old female and 2 year old male) started showing the same symptoms (gastro with fever) and were then also admitted.

Discharged 3 days later and told to monitor and feed only the royal canin gastro food (wet pouches). All 3 refuse to eat them, but they will eat chicken.

We are on meal 4 at home and they will eat chicken (plain, no bones) and even a bit of the wet food they used to eat (high quality tinned) with the chicken, but they refuse the gastro food.

My vet is adamant they need the gastro food but like...any food is better than no food IMHO.

All three were stressed the heck out by all handling etc, and one in particular who is an ex-street cat (rescue adopt from reputable org), came home covered in her own pee (she was anxious in the carrier this is not the first time she has done this, we reduce the amount of time she needs to be in it, or handled as much as is possible).

Has anyone had a similar experience of cats not willing to eat the gastro food? I know they have nausea so plain chicken is palatable but everything else is yuck for them right now.