GTA V had Mt Chiliad. RDR2 had the Grizzlies. GTA 6 has - Florida. Florida's highest natural point is Britton Hill, at 345 feet. There will not be mountains.
We look at what elevation there actually is on the map and what 'high ground' means in this context.
Why there aren't mountains
Florida is flat. The state is a sedimentary platform that sees minimal changes in elevation. If the GTA 6 map is going to be accurate to Florida, there can be no mountains. Every trailer so far has shown flat-to-rolling terrain.
Leonida's definition of a 'mountain' may be closer to a hill or a ridge - 100-200 feet of relief at the most. That isn't a mountain by GTA V standards.
What might pass for elevation
Inland sandy ridges. Man-made hillocks surrounding developed areas. Dirt piles at construction sites. Perhaps one or two quarries. None of these offer the up-and-down experience that GTA V's Mt Chiliad provided.
'Mount Kalaga' has come up in some community discussion. Whether it's official or fan-created is one of several things we're still not sure about, so we discuss it tentatively.
What the lack of mountains changes
Flying is different - you can't BASE jump off mountains. Photography is different - you don't get those Vinewood Hills angles overlooking LS. Story-wise, you won't get those classic 'last stand on the mountaintop' set pieces.
Instead, there will likely be water-based and rooftop-based set pieces - boat chases, bridge showdowns, high-rise sequences. A different vertical vocabulary.