Technically there is no such thing as a "gui relational database", since a database is not an application and therefore has no interface ;-). I think what you mean is a DBMS like Access, since there are any number of database interfaces which could be considered graphical, web-based systems being the most prevalent in my experience. There are even graphical DBMS's for MySQL that have been around a while, but aren't really that intuitive and are more for administration than something for the end-user.
For Access-like DBMS's there are several to consider. My favorite is Rekall. It is KDE-based and is now licensed under the GPL. In it you can create forms, reports, and all the other Access niceties, and it uses Python for its scripting language (as opposed to VBA). There is also Knoda. I've never tried it out, but it is KDE-based as well, and seems to be maturing. Both of these apps support multiple Database server back-ends (such as MySQL, PostgreSQL, etc.), and I Rekall has its own XBase system for personal databases as well (not so sure about Knoda, but it may be able to use SQLite as a backend, which would accomplish about the same thing). If you want to go the proprietary route StarOffice has its own database system, which I'd assume uses StarBASIC for scripting (which, if true, makes converting from Access all the easier). Oh yeah, and there's also Kexi, which will be part of KOffice in the next major release after 1.3. It is currently available as a Beta release
Here's some applicable links:
Rekall -- http://www.rekallrevealed.org
Knoda -- http://www.knoda.org
StarOffice -- http://wwws.sun.com/software/star/staroffice/
Kexi -- http://www.koffice.org/kexi/