How much could sequential scan gain by GPU?
You'd better ask this question on the hackers mailing list, there has been some debate about this subject in the past. The PostgreSQL developers can tell you much more about it.

http://archives.postgresql.org/pgsql-hackers/

To get good performance using a GPU for PostgreSQL, you need some form of parallel processing in PostgreSQL. This is something PostgreSQL doesn't have at the moment (for CPU nor GPU), but Greenplum already implemented this in their PostgreSQL-product for CPU's. Parallel processing is on the ToDo list, but it looks like nobody is working on it at the moment. Ask the PostgreSQL hackers about the current status.