AQL is a vendor-specific multi model language that lately has picked up some ideas from openCypher, like pattern matching.

GQL is a project for creating an International Standard language in the process of being created by experts from many countries that represent the interests of multiple vendors.

I haven't had a look at GQL yet but the obvious one would likely be that as ArangoDB aims to support native multi-model, AQL is not just a graph querying language.

Graph databases generally only consider edges between vertices (identified by IDs) and those edges typically only have a handful of attributes. In ArangoDB edges and vertices are stored as documents, so they can be represented as arbitrary JSON objects.

So if you want to work with a mix of data or want to access your data in different ways, I wouldn't expect GQL to support that whereas with AQL you can mix and match your queries even within the same request.

Full disclosure: I'm an external contributor to ArangoDB but this is my personal take.