Nothing is jumping out at me. If the gables have no holes then that's easy, else check your header or lintel capacity. The floor, girder and roof trusses are engineered by the supplier. Understand what I was saying wrt the joist spans and deflections, you'll want fairly stiff joists and girder at long spans. This is a no tile job. I'm assuming the roof trusses are an attic type with a loft floor clear spanning the 28', no truss loads landing on the main floor. Read the section in the IRC on foundation construction and reinforcement. You'll probably want double jacks on headers over 6', look at the header and girder table, chapter 5 I believe for sizes and number of jacks.

Some quick math, the girder spans 28' and supports half of each joist on each side (the gable foundation walls support the other half of each joist), so tributary width bearing on the girder is around 20'.

Look at the post first, same thing, the post supports the center half of the girder 7' each way from the post along the girder, 14'

Area supported by the post is then20'x14'=280 sf. Multiply by dead load of the floor 10psf+ 40psf live load. 280 sf x 50 psf design load= 14,000 lbs, the post, at its length, has to be capable of resisting that load without buckling. Check the post capacity on the sticker.

The footing under the post likewise. Assuming 2,000 psf soil bearing capacity 14,000 lbs/2,000 psf=7sf of footing area under the post. Square root of 7 is around 32" square for the post footing in 2,000 psf soil. To resist "punch through" of the post through the footing it needs to be thick enough to maintain an imaginary 45 degree angle from edge of the 4" post to bottom of footing (32-4)/2=14" thick. So spitballing it, a 32" square x 14" thick post footing. You may have better soil numbers to plug in above.

How big is the girder. 14' span wall to post x 20' trib width=280 sf x 50 psf =14,000 lbs, same solution but now we are looking at the area between wall and post on either side of the post.

Assuming LVL's for the girder, a triple 14" deep LVL girder should be in the ballpark. The supplier will do all that just some quick numbers to play with sizes in your drawing, headroom, post length, etc. Download the I joist catalog for your supplier, I suspect you'll be using BCI's but check. You should be able to look at the choices and get close, start with 14" options if you want a flat ceiling below.

Now with those elevations sketched in...

I've just had you criss cross the framing between floors, solve one problem, create another, I'd play with stairs next.