From what I've seen in Roger Cicala's review comments, the GM is excellent in the 16-24mm range.
A nice advantage of the zoom is being able to shoot the entire job with one lens. It removes one headache from the project.
The GM has a lot of aperture blades, so the sunstars are messier - the FE 16-35mm has better looking sunstars. Also the F4 model is a lot more affordable. Still, I'd want to give the GM a solid try because it could possibly replace your Batis for astro.
Sell the Batis, but the Loxia 21mm is too good to ever sell imo.
My attitude is buy with a sincere expectation of keeping it, but returning the item if it doesn't measure up to your realistic expectations.
In case you didn't see this FM thread on the GM: http://www.fredmiranda.com .../0?keyword=16-35#14214712
Really appreciate your inputs David!