how many cores uses gbak (firebird) on restoring?
As indicated in the tickets linked by Val Marinov in the comments (CORE-1365, CORE-2992 and CORE-3958), a restore in Firebird is currently entirely linear and will only use a single CPU core. There is nothing you can do to change this, short of sponsoring/funding an improvement to Firebird to get this into a next version with priority.