PostgreSQL GIN pg_trgm default operator class
you can update the pg_opclass table and set the default.

update pg_opclass set opcdefault = true where opcname='gin_trgm_ops'


pg_trgm doesn't do this by default as you might have/want a different GIN default.

select * from pg_opclass where opcname = 'gin_trgm_ops';


than you can create an index:

create index ON table USING gin (field); -- it will use the new default