They're heavily based off of the Roman Empire and there's quite a lot of similarities between them. I've said this before in a different thread but even to this very day in the real world an immense amount of our culture is directly influenced by what the Romans did be it in terms of architecture, art, entertainment and even the method we use to tell time. They were, however, very driven in regards to wanting to conquer more and more territory and establish dominance. The irony is that even though the Roman Empire fell they technically succeeded - because even in the modern day we're very reliant on what they did for us.
It's pretty similar with Garlemald. Yes, Allagans are ultimately responsible for Magitek but Garlemald are the ones to have brought it back - and now Eorzea is quite rapidly embracing it as well as a direct consequence of Garlemald doing so.
They're certainly morally grey, there's no doubt about that - but we don't know a whole lot about their motives as of yet beyond them seemingly being well intentioned extremists. Yes, they resort to harsh methods but in a setting that has a lot of awful stuff going on I feel like a lot of people just rally against them as the 'easiest' targets.
Still, I'd like to see more Garleans who are similar to Drace and Gabranth from FFXII who are loyal to their homeland and people and aren't traitors - and I'm hoping that we'll eventually see that much like we found that Ishgard wasn't fully rotten to the core like many suspected.
One of the biggest issues with these sort of things, however, is that a lot of people are prone to viewing stuff as either 'black' or 'white' even if they claim not to. I mean, for those familiar with the likes of The Walking Dead and Game of Thrones there's a lot of characters who are pretty morally dubious by our standards. By the standards of the settings themselves, however, their actions and behaviour are...pretty understandable, really. I won't spoil anything since they're not FFXIV based stuff but...yeah. Hopefully someone gets what I'm rambling on about, eh?
Oh, and because I feel like it needs to be highlighted:
For all the criticism of Garlemald oppressing and killing those who boast different ideals to themselves...the WoL and their allies are prone to embracing the exact same mentality. I mean, think about it - how many lives have been snuffed out by the WoL and their friends? Did some have it coming? Arguably, but what about the majority who were just following orders or were loyal to their friends, family and causes?
I'm hoping we see a scene in the future where the WoL strikes down an opponent only for it to be revealed that their opponent was standing in their path to allow a friend or lover to escape. Because let's be honest - that's the sort of thing that is very, very likely to be occurring during the countless battles that have transpired thus far.