Switch database to single user mode, like
gfix dbalias -user SYSDBA -pass ***** -shut single -force 0

Then run this batch file:

:test
gfix dbalias -user SYSDBA -pass ***** -mend
gbak -b -ig -g -l "127.0.0.1:dbalias" "C:\backup.fbk" -user SYSDBA -password *****
del "c:\backup.fbk"
goto test

In 16 runs I get 3 instances of either gbak or gfix reporting that the database is offline. This makes it hard to run gfix/gbak and perhaps other tools sequentially in a script. It would seem that sometimes the engine does not release the SYSDBA connection in a timely manner.