> But why does gpre map LIKE to something it shouldn't do?

It has nothing to do with gpre.
LIKE operator is able to use index only if the pattern does not start with wildcard:
myfield LIKE 'abc%'
If you have command that has its pattern as parameter
myfield LIKE ?
than during prepare of such command FB has no idea what
the actual value will be when executing it (whether is will start with wildcard
or not), hence the plan can't use index.

Ivan