What you describe is actually pretty typical behavior of parrots who have picked a favorite person (you!) seen as their mate and will protect you now from danger (other people!).
While some people don't mind this behavior because they may not have many visitors, in your situation it should not be encouraged because you are living with other people and therefore you need your bird to accept others. It is not fair for these people living with you to be bitten and you may face them telling you to get rid of your bird if it persists.
Cockatoos are a whole are very challenging birds, which is why about 75% of them get rehomed after 2-4 years by their owners. There are tons in rescues because of their over-bonding tendencies, their toddler demeanors that they never grow out of and their tendency to develop behavioral problems if not well socialized are just a few of the challenges faced with cockatoos of all sizes. It takes a special person to own a cockatoo.
I don't own a galah or cockatoo but hopefully other owners can chime in with their tips for you.
To keep your cockatoo from over-bonding with you, you should not be petting your bird anywhere but his head. No close cuddling, petting along the back, along or under the wings, as this can and will stimulate a cockatoo sexually. They begin to think you're its mate and then get frustrated and aggressive when they can't mate with you.