Your indexes and operators are not compatible. You have added a btree index on md5 function result and are not using md5 in your query, and also using LIKE operator not one of the supported ones. I believe it might use a btree operator (plain value, not md5 result) if you are always searching for "string starts with ____ but I don't know what it ends with" but you can't possibly use a btree index where you are putting a wild card at the front.
https://www.postgresql.org/docs/9.5/indexes-types.html a gist index operators supported- https://www.postgresql.org/docs/9.5/gist-builtin-opclasses.html Here's a whole page on full text search, it would be worth a read- https://www.postgresql.org/docs/9.5/textsearch-tables.html#TEXTSEARCH-TABLES-INDEX
