Hi,

>By using gfix, can I bring database to single user mode and attatch only
>once at a time?

YES

you may use:

gfix -shut full -force 0 //shutdown and no connections alowed

and than

gfix -online single // bring on-line just for 1 connection from
sysdba/owner

later you may use

gfix -online [normal] to bring the database to normal online state


regards,
Anderson Farias