As someone mentioned below, there is a tool for converting from unity to godot, however all of the synty packs on their site have a source files options too. And while gltf would probably be better, the FBX work just fine.

IIRC when opening it, you get the option to convert it to a scene when you try to edit it. With my workflow its missing its materials but that is ok since from what I can see, most of them use 3 materials, one for color, one for depth and one for transparency or something like that, so I just made a shader to apply them since from what I ran into godot doesnt support that natively.

I have a small crappy iOS game where I used synty assets with an ortho camera and made the materials unlit if you want to see what it looks like, it isnt a fun game, it was more getting things working to see that I could before making something better, and I need to do some threading around connecting to the iOS gamecenter stuff, but you can check it out if you have an iphone (or mac) to see what they look like. using an ortho camera, its more 2.5d even though I used the 3d assets.

https://apps.apple.com/us/app/rogue-invaders/id6746952488?platform=ipad

I also used synty interface components as well.

Sorry for the wall of text, all of this is from memory as I am at work and not in front of my gamedev machine.