Pull up all your numbers in a found set. Use Replace Field Contents with its serial number option and it can renumber the entire set, but:
If this account number field is used to link your account records to records in other tables, such renumbering will pretty much make a hash out of your database by screwing up the links to your related records data.
Since auto entered serial numbers, if they are used as match fields, should be meaningless. The fact that you're even able to see that there are gaps in the numbering indicates to me that something's amiss. The fact that there are gaps should mean nothing.
That would depend on how Gary Lewis is using these numbers. For auditing purposes, it's often necessary to show some kind of ID on documents and often the presence of a "gap" creates issues with the auditors. They might not even be match fields used in a relationship though by the description, it would seem that they very likely are used to match to related records--such as transactions logged against each account.