try below:

1. connect to mysql by hand ,to check the gforum database have success connect ??

2. check /cgi-bin/gforum/admin/defs/database.def
check the database connect setting is change to new databases??

3. check system the module DBI and DBD::mysql work ok??

4. in command line run
perl admin.cgi
to see it work ok??

5. if you use new mysql server,
maybe use "mysql studio" function "database trans" to make all copy old database data to new mysql.

my MSN: perlchina_at_hotmail.com