Global fields on a hosted file are specific to each user, so there is no problem with having more than one setting happening simultaneously. Each user will see only their own global settings. Once the globals are exposed to the user to modify as required, all is well.
If your point is to prepopulate the fields with different settings, then you will need to store the different settings somewhere and then populate according to how your script navigates. You would have to store each set of settings in separate records, and then work out a method of each user selecting the correct record to use. Shouldn't be hard—you could base the selection on the user account, and store the user account in the same table as the settings. Something like that.
Once the G fields are populated by User 1 if User 2 opens the database and the script populates the same G fields. Will the two users see different settings on the same field?
On a HOSTED database, the global fields are User-Session based.
You are correct, what User "A" sets will be unique while "A" is in the file where the global field resides. If at the same time, User "B" enters data, it will be different and "A" will not see what "B" entered.
If there is no value set in the field (before being hosted), then the field will be blank the next time "A" or "B" opens the database. If there is a value in the global field (before being hosted), then both users will see that value, but can independently change it. When either session is over, the global reverts to "empty" or "filled" (before hosting) for that user.