From the IBPhoenix KnowledgeBase:

Question:
Syntax for backing up to a tape drive is not provided with the InterBase
Document set.

Answer:
Backup: gbak -b mydb.gdb /dev/rmt/0 <--Tape drive device name
Restore: gbak -c /dev/rmt/0 mydb.gdb

d:\ib\bin\gbak -b -t -y \gbak.out d:\dbs\mydb.gdb \\.\tape0 -
user "SYSDBA" -password "masterkey"

Regards
Paul Beach
Main Tel (UK):+44 (0) 1844 354465
Mobile: (UK): +44 (0) 7764 188603
http://www.ibphoenix.com