It’s possible that it’s the type change, anonymous, although you shouldn’t rule out a number of other possibilities.
Luckily, there’s a handy way of figuring out whether or not it is because of your ninetales’s type change. Have your gabite enter the room shortly after your ninetales leaves it. If she shivers, then she’s reacting to either the cold or residual fairy aura, and thus, it is, without a doubt, a problem with your ninetales’s type. While, yes, your vulpix, as an ice-type, may have triggered a slight drop in temperature of the air immediately surrounding his body before he evolved, keep in mind that ninetales are far stronger and possess a far more intense command over their native elements. As such, it wouldn’t surprise me if your gabite is only just now feeling discomfort or even abject fear of being around your ninetales.
If this is the case, then resolving it may be as simple as sitting down to talk with your gabite and help her acclimate to your ninetales (all while reassuring her that he is, at his heart, still the vulpix she befriended, of course). If it’s the cold that’s bothering her, then you may consider giving her a sweater to help her tolerate his abilities. Many independent artists online knit wonderful sweaters for pokémon in a number of designs, but if you’d prefer something a bit easier to come by, you may be able to find sweaters in your gabite’s size at the local pokémart. The latter aren’t normally sized to fit gabite (whereas sweaters by artists can be custom-created), so be sure to get a sweater for “medium-sized” pokémon.
As I’ve said earlier, however, this is just one possibility, and it’s certainly important to take into consideration other potential causes. For example, if your gabite looks more irritated or panicked than physically uncomfortable, there may have been an argument or even a fight between your ninetales and gabite when you weren’t looking. If that’s the case, then it’s important to serve as a mediator between them. Sit them down and try to communicate with them. Try to figure out how their differences started and work with them to repair the relationship between them. (Alolan ninetales can sometimes be mischievous, so bear that in mind as well.) Have them engage in team building exercises such as double battles in order to get closer. Ensure that you’re treating them as equally as possible to avoid making one or the other jealous.
The main thing is that it’s never a good idea to jump to the conclusion that your pokémon are rejecting each other due to a change in type after evolution. While that’s certainly a common problem, it’s not the only one. Be absolutely sure that it is the reason behind your team’s differences (through testing and observation) before attempting to treat the issue.
Best of luck, anonymous!