http://www.postgresql.org/idocs/ is a good place to query PostgreSQL 
documentation.

1) Create trigger
http://www.postgresql.org/idocs/index.php?sql-createtrigger.html

CREATE TRIGGER trigger [ BEFORE | AFTER ] [ INSERT | DELETE | UPDATE [
OR ... 
] ] ON relation FOR EACH [ ROW | STATEMENT ] EXECUTE PROCEDURE procedure 
(args);

2) Drop trigger
http://www.postgresql.org/idocs/index.php?sql-droptrigger.html
DROP TRIGGER name ON table

3) Alter trigger
Not supported. Per discussion on hackers, clients should handle
DROP/CREATE as 
there is no need to preserve object ID.

4) Additionnaly, PostgreSQL 7.3 allows to enable / disable triggers.

Triggers are now reported when fetching meta data and storing it in the GdaMetaStore object, this is a V4 API. Closing the bug.