Garlean Inability to use magic.

[SannaR]

Well Zenos is a special case considering who his great grandfather is. So, he's already cheating. There are some who are able to use magic. They're an interesting race as if I have my lore correct their ancestors were those who lived in the clockworks city of Galug. Back during Ramza's time. Which I want to say could use magic? Then they just happen to be chased from Corvos the area that G'raha is from. You know the people who still had Allagan royal blood being passed down.

Thancred was able to use magic and his own aether before being altered by the use of the spell Flow. He can sort of use his own aether still, but it's harder. It's why he nearly died during his fight with Ran'jit.

[MikkoAkure]

Well Zenos is a special case considering who his great grandfather is. So, he's already cheating. There are some who are able to use magic. They're an interesting race as if I have my lore correct their ancestors were those who lived in the clockworks city of Galug. Back during Ramza's time. Which I want to say could use magic? Then they just happen to be chased from Corvos the area that G'raha is from. You know the people who still had Allagan royal blood being passed down.

The people of Goug could not use magic. The modern Garleans descended from 7 tribes of the same race of people, one of which are those from Goug. They all happened to converge on Corvus after Goug was destroyed and many eras after G'raha's ancestors had settled the land. Whether one or a few tribes of proto-Garleans were also settled in Corvus going back to the time of the Allagans, we do not yet know, but the Allag royal blood was only being passed down within a family of Miqo'te so they won't have anything to do with it.

As for the original question, this is what Koji Fox had to say a while back:

This is an easy question. They are genetically unable to. It’s in their DNA, because they have all of this knowledge. They basically go and take over a bunch of countries, but they don’t destroy the nations when they go and take them over. They kill the leaders and then let the nations remain as they are, because they realize that’s a better way to have them remain loyal to the Garleans and not have an uprising. Because of that, the Garlean Empire has access to all of this information, but even with access to all of that knowledge on magic, they still can’t use it because it’s in their genetics. That’s why they’ve built up all of that machina to fight the magic that’s so powerful in Eorzea.

As far as Emet-Selch goes, he cheats and can cast magic on the Source due to the benefit of being an Ascian. He is basically a ghost of an Ancient possessing the body of a Garlean who was once his own person and had no capability to cast magic on his own. Whenever we interact with Emet-Selch on the First, he is not actually in the body of an Garlean, he possessed some random person and then molded his body to look like Solus because that's a form he's comfortable with.

[SannaR]

The people of Goug could not use magic. The modern Garleans descended from 7 tribes of the same race of people, one of which are those from Goug. They all happened to converge on Corvus after Goug was destroyed and many eras after G'raha's ancestors had settled the land. Whether one or a few tribes of proto-Garleans were also settled in Corvus going back to the time of the Allagans, we do not yet know, but the Allag royal blood was only being passed down within a family of Miqo'te so they won't have anything to do with it.

Yet Ramza if he is of Goug was a WoL that would imply that he'd have to be able to use magic. In the fight against Ultima they create a magical barrier. Anyway if the Allagans were anything like the Machi they would have most likely taken similar steps to continue on some how. That and we also get told that one of the prominent families that came from Corvos is the house of Darnus.

[MikkoAkure]

Yet Ramza if he is of Goug was a WoL that would imply that he'd have to be able to use magic. In the fight against Ultima they create a magical barrier. Anyway if the Allagans were anything like the Machi they would have most likely taken similar steps to continue on some how. That and we also get told that one of the prominent families that came from Corvos is the house of Darnus.

Ramza's not from Goug. Mustadio in the original FFT was but this one doesn't have a third eye. He's obviously included in this band of WoLs as fanservice for FFT players since he was a member of that group so you could probably pass this one off as a half-Garlean, but we never get the chance to ask.

Regardless, all the Garleans on the ship talk about the people of Goug being ancient ancestors of Garleans if you talk to them after the lighthouse and at least one mentions the Goug inability to control magic.