I think you need to add the "default" maemo repository (URL "http://repository.maemo.org/", not extras). I don't know why it isn't already set up on the default firmware. Here are the details:

Web address: http://repository.maemo.org/
Distribution: bora (or "gregale" for 770 users)
Components: free non-free

Alternatively, you can just install GDBM manually via this link into the repository:

libgdbm3 for gregale (770)
libgdbm3 for bora (N800)

I've uploaded libgdbm3 to the gregale and bora extras repositories, so as soon as they're updated, you won't need this extra step - the .install file should grab the libgdbm3 dependency automatically.

P.S. I really hate Maemo's repository situation.