There are at least 2 types of gap: Functional and Knowledge
Gap management is something that starts in the selection phase in that after the selection evaluation some gaps will already be evident in the selected solution and should have been documented by adding to the initial gap list.
But the rubber hits the road in the implementation phase when gap management becomes a key concern of the delivery team.
Early emergence of dozens of gaps will suggest the selection decision was wrong and may suggest a project pause and review. I have been on projects where this triggered a partner change but it could also trigger a solution change.
Fewer gaps may appear to validate the selection decision but may also be indicative of inadequate analysis activity that will store up problems for later. If you don't know what you don't know then your solution may be deficient.
There are at least two types of gap:
(1) Functional gaps are 'holes' in the functional solution and the decision is whether they need to be filled, filled now or filled later (post go-live).
Functional gaps are usually closed in 3 ways:
By acquiring third party add-ons
By defining acceptable workarounds
(2) Knowledge gaps are 'holes' in the partner/vendor knowledge of your business/processes or your knowledge of the ERP solution.
Knowledge gaps are usually closed in 3 ways:
By formal analysis sessions
By process walkthroughs or playbacks
By continued exposure to the system in day-to-day use
Lesson: Gap management is an important implementation activity.