The search query uses the LIKE operator which requires a sequential scan (it can't use the already existing B-tree index). This new index (trigram) avoids a sequential scan of the builds table when the LIKE operator is used.

Here is the analyze of a request on the builds table with this index:

 explain analyze select * from builds where drvpath like '%k3r71gz0gv16ld8rhcp2bb8gb5w1xc4b%';
                                                             QUERY PLAN
 -----------------------------------------------------------------------------------------------------------------------------------
  Bitmap Heap Scan on builds  (cost=128.00..132.01 rows=1 width=492) (actual time=0.070..0.077 rows=1 loops=1)
    Recheck Cond: (drvpath ~~ '%k3r71gz0gv16ld8rhcp2bb8gb5w1xc4b%'::text)
    ->  Bitmap Index Scan on indextrgmbuildsondrvpath  (cost=0.00..128.00 rows=1 width=0) (actual time=0.047..0.047 rows=3 loops=1)
          Index Cond: (drvpath ~~ '%k3r71gz0gv16ld8rhcp2bb8gb5w1xc4b%'::text)
  Total runtime: 0.206 ms
 (5 rows)