I have just committed a series of patches that provide support for enum types in the
btree_gist standard extensions. This is something I first started work on about a year ago. It turned out to be more involved that I had first thought it would be, as it requires some additional core code due to the way that enum comparisons work, which is a bit more complex than for most data types, and involves use of PostgresSQL’s internal caching mechanism.
The practical upshot of this, however, is that starting with PostgreSQL 10 you will be able to use enum columns in exclusion constraints. That’s something that could be very useful – I started this work when I found, somewhat to my surprise, that it wasn’t possible.