Operating system: Linux-4.15.0-52-generic-x86_64-with-debian-buster-sid 64 Bits
Graphics card: GeForce GTX 1060 6GB/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 430.26
Broken: version: 2.80 (sub 74), branch: master, commit date: 2019-06-29 14:01, hash: rBf566212aa237
Description of issue
I don't know if this is intentional or not, but currently custom gizmos that define their own geometry don't appear antialiased in the 3D view and they also don't make use of the alpha channel defined in their corresponding properties, namely alpha and alpha_highlight.
I've found that the issue is solved when bgl.GL_BLEND is added to the gizmo draw method.
Here I show the example from the builtin template gizmo_custom_geometry.py for lamps, before and after adding bgl.GL_BLEND.
It seems that bgl.GL_POLYGON_SMOOTH and bgl.GL_LINE_SMOOTH are already in use internally for the gizmo, providing antialiasing, which is great, so I suspect that bgl.GL_BLEND may be missing from inclusion, and I don't see any reason why it shouldn't be used by default.