Though Postgres provides nonblocking read/write access to table data, nonblocking read/write access is not currently offered for every index access method implemented in Postgres.
The various index types are handled as follows:
Share/exclusive index-level locks are used for read/write access. Locks are released after statement is done.
Share/exclusive page-level locks are used for read/write access. Locks are released after page is processed.
Page-level locks provide better concurrency than index-level ones but are subject to deadlocks.
Short-term share/exclusive page-level locks are used for read/write access. Locks are released immediately after each index tuple is fetched/inserted.
Btree indices provide the highest concurrency without deadlock conditions.
In short, btree indices are the recommended index type for concurrent applications.