Ignore about everything the ppl above have said.
Converting at 90 is indeed a good point to make them DP gunners.
However the moment you class them, they will most likely not have reached the reload cap yet, which is the reason for the delay in classing them. You'll need more levels and/or more vets if you want to reach it sooner.
As a standard 100/109 vets and a good amount of experts is recommended for gunners, but if you feel lucky I wouldn't hesitate to go (much) further. Reaching that cap earlier is always nice, and it might even allow you to shave of some recruits from the gunners when you go over the reload cap ( not exactly known for ijn 4.7", but I guess it's between 2.5-2.7m TA)
Especially with this weeks veteran conversion event going on I would try to convert every expert you have. 300% vet conversion is as high an event as $DE goes.
Since you plan on using AA ( obviously), regaining the experts shouldn't be too much of a problem.
As for gunners not "needing"high amounts of vets on gunners, it's both correct and false.
It's correct in that most gunners reach their caps rather easily ( except DP gunners!!!). so you don't need high vet counts to reach it. But it is false in that with more veterans and some experts you can make gunners considerably lighter. Also for a lot of gunners it doesn't hurt them much to try for more veterans, their ability score and number of veterans will allow them to reach the caps still, despite having almost no experts. When you're shooting for high vets on gunners, do some calculations on how many experts you should leave to still keep them reload capped ( acc cap is lower and will be reached automatically then).
Unless you're loaded don't buy experts to try for high vet gunners though.