It has really nice colours, but it doesn’t look good in Unreal due to the size.
How can I make it look nice somehow or use it as a mask for Quixel textures?
So forget about normal maps, even if you add it it will make no difference from ground level. Use albedo as last/top level color correction.
You should export terrain/texture types from gaea and use that to mix layers for different terrain textures. And those terrains should have normal map that adds local variation and detail.
When all terrain materials are mixed you modify color with albedo texture.
ps.
In case you did not know: in gaea you can adjust height ratio (range) to size of map, try to make map size there comparable to map size in unreal.
All features like hills rivers etc will have proper/natural sizes, however that kind of does not work for games, distances appear reallife and 500m takes minutes to walk. So if you making game, test size of map, natural/real feeling of it etc, it is very hard to balance all, esp slope of terrain vs size and height of map.