Normally I would suggest the additional module btree_gist, but the type uuid is not covered by it.
In theory, since a UUID is a 128-bit quantity (per documentation), the most efficient way would be to convert it to two bigint or float8 for the purpose of the index. But none of these casts are defined in standard Postgres.
I found a stab in that direction in the pqsql-hackers list, but it seems unsuccessful.
The remaining option is a functional GiST index on the text representation:
CREATE INDEX idx_leaderboads_values_gist
ON leaderboard_entry USING gist (id_leaderboard, cast("value" AS text));
To make use of this functional index, queries must match that expression. You can use the shorthand "value"::text in queries (but not in the index definition without adding more parentheses).
Aside: do not use value as column name it's a reserved word in standard SQL.
The question is: why do you need the GiST index. The best solution depends on the objective.
