We support GSS for [authentication](https://github.com/npgsql/npgsql/blob/b36710592a58d4efb33dc64f3b9a763dd4589abe/src/Npgsql/NpgsqlConnector.Auth.cs#L221), but not as a [session encryption method](https://www.postgresql.org/docs/current/protocol-flow.html#id-1.10.5.7.12), introduced in PostgreSQL 12. This shouldn't be too hard to implement via [NegotiateStream](https://docs.microsoft.com/en-us/dotnet/api/system.net.security.negotiatestream?view=netcore-3.1).