:: host/port:database
::
:: ie fluffy/3051:C:/Database/Bunny.fdb
:: or yoda/1111:/var/firebird/jedidb.fdb

Doesn't work:

Executing : C:\Program Files\Firebird\Firebird_1_5\bin\gbak.exe
With Command Line : -B -USER SYSDBA -PAS <password>
"xx.xx.com/9999:D:\data\data.gdb" "D:\data\data.gbk"
gbak: cannot open backup file D:\data\data.gbk
gbak: Exiting before completion due to errors
Program returned code : 1

Not the real paths or ports, but changed for public postig only.