ERROR: data type text[] has no default operator class for access method "gist"
smlar Extension

I was able to create a gist index on a text[] column using the smlar extension (written by Teodor Sigaev of text search fame)

# git clone git://sigaev.ru/smlar.git
# cd smlar/
# PATH=/usr/pgsql-9.6/bin:$PATH make USE_PGXS=1
# PATH=/usr/pgsql-9.6/bin:$PATH make USE_PGXS=1 install
# psql

postgres=# create extension smlar;
postgres=# create index on bar using gist(f _text_sml_ops);