If I am running 28 wide inner tubes, and 23" Grand Prix 4000s2's over time will my tires stretch out cause thinking of switching to 23" inners?

Tires always stretch (which many of us think is a good thing), but it won't be because of the tubes. You're just riding with a few extra grams and improved puncture resistance.

A caveat here - when you take it off, it will be more susceptible to pinch flats, because the tube will be stretched - might be good then to go with a new smaller tube.
Carry a spare.

It's beyond my comprehension that your tire would stretch enough to make a difference with the tube since the tube can expand significantly larger than the interior of the tire. This particular tire has tire cord embedded in the tire (330 threads per inch) and if I interpret the technical specifications here correctly, there may be three layers of tire cord continental bicycle Grand Prix 4000 S II. If it were just pure rubber, I could go along with significant "tire stretch" but it is not.

When I choose tubes for my 700C X 23 tire, I use the one that is suitable for the size that accommodates 18-23 mm instead of a straight 23 mm because it makes it much easier to install the tube and really doesn't matter otherwise. Pinch flats are less likely with any tube if you maintain proper pressure in the tire and avoid hitting stuff lying in the road.

Tubes don't resist the air pressure, tires do. Tubes just stop air from sneaking out of gaps. If you pump 100PSI into a tube, it doesn't matter if you've got a 23c tube or a 28c tube, the tire is basically going to be feeling 100PSI. Actually, it's not possible to pump very much air into a tube if it's not in a tire, as it'll quickly stretch out as it inflates and then explode.

We don't use tiny narrow tubes in super wide tires because an extreme amount of stretch isn't a good idea. We don't use super wide tubes in extremely narrow tires because they weigh more and it could be difficult or impossible to seat the tube without pinching. But for small differences? Whatever.

You can fudge tubes with respect to wheel diameters a little, too; a 700c tube will work fine on a 27" wheel, even though trying to fit a 700c tire on a 27" wheel simply will not do.

I'd probably purchase 23c tubes as spares in the future since they're a little easier to install in a 23c wheel, but if the 28c tube is working, I wouldn't worry about replacing it until it flats.