someone asked on the first page for a source that the cherem rabbeinu gershum was renewed after the expiration date specified by rabbeinu gershum. if it is expired, then that my be a solution to use on a limited basis.
another possibility is to emulate the chasidish shidduch practices, since they dont have an age gap. many litvish rabbonim advocated having both boys and girls marry at a young age. if litvish boys would get married starting at 18 (similar to the chasidim), they would almost automatically be marrying close in age girls. (after all, how much younger than 18 can the kallah be?)
2 potential solutions right here!