The key to minimizing gaps in your defence is to create as many tackle zones as possible. The more tackle zones you have on the field the less likely your opponent is of making breaks through your defence.
Try to spread your defenders effectively and try to limit the amount of overlapping Tackle Zones. Doing this will give you a good defensive barrier against any attack.
EXAMPLE: The Italian team has spread their defense across the field and made very effective use of their tackle zones with no gaps!