If Caesar tells us that Aquitania is bound by the Garonne, the Pyrenees, and the ocean, what else do we learn from spectat inter occasum solis et septentriones?
You raise an interesting question. I think, however, that you are making some unspoken and unwarranted assumptions.
Firstly you read the text with your modern understanding of disposition of the land Caesar describes. Secondly you do not engage with Caesar’s politics.
You seem to imagine that Caesar’s contemporaries were familiar with the geography he describes, but in fact according to Cicero the reverse is the case. See this interesting article:
https://research.ncl.ac.uk/histos/documents/2019AA03JohnstonRewritingCaesar.pdf
Caesar’s description which on the surface is a strictly geographical one has a political purpose. No one had previously described the region. He was marking it out as his creation which “he was going to subdue militarily” (Johnstone).
I think also that it is difficult to follow what you are saying because your quotes are out of context. Here are some quotes from the Loeb:
Eorum una pars, quam Gallos obtinere dictum est, initium capit a flumine Rhodano; continetur Garumna flumine, Oceano, finibus Belgarum; attingit etiam ab Sequanis et Helvetiis flumen Rhenum; vergit ad septentriones. Belgae ab extremis Galliae finibus oriuntur; pertinent ad inferiorem partem fluminis Rheni; > spectant in septentrionem et orientem solem> .
The separate part of the country which, as has been said, is occupied by the Gauls, starts from the river Rhone, and is bounded by the river Garonne, the Ocean, and the territory of the Belgae; moreover, on the side of the Sequani and the Helvetii, it touches the river Rhine; and its general trend is northward. The Belgae, beginning from the edge of the Gallic territory, reach to the lower part of the river Rhine, > bearing towards the north and east> .
Aquitania a Garumna flumine ad Pyrenaeos montes et eam partem Oceani quae est ad Hispaniam pertinet; > spectat inter occasum solis et septentriones> .
Aquitania, starting from the Garonne, reaches to the Pyrenees and to that part of the Ocean which is by Spain: > its bearing is between west and north> .
So the question is where are these bearings of north east and west from? Perhaps its from the Roman province of Gallia Narbonensia (Horum omnium fortissimi sunt Belgae, propterea quod a cultu atque humanitate provinciae (the Roman province of Gallia Narbonensia, formed about 121 b.c.) longissime absunt). So in effect Caesar is describing unknown (less well known) regions with respect to their bearings from civilisation - Gallia Narbonensia. To the North be dragons. But more importantly the act of defining these regions in this complete way is a way of showing his mastery over them.