I'm working for a group of companies who want to share GALs. I know I could do a manual export and import of each - is there a way to automate this natively? (all are on EXO) without buying a third party tool?
there's no built-in capability in EXO. Your best option is to purchase a third-party synchronization tool, or do it yourself. The benefits of such a tool are that you don't have to worry about updates etc. (they are handled automatically). There's a few tools out there that can sync cloud-to-cloud. So, no need for an on-prem server...
I think you should make up your own mind. Try a few an decide on which fits your budget/needs/etc.
This being said: have worked with NetSec's GalSync, Binary Tree's solutions in the past. And they work really well. Way back, I also used Quest's tools. But not sure what they still offer today. The company has been bought/sold many times :)