Difference between the User Exit & Gap analysis.
Both are quiet a different and has a small relation.
User exits are standard gate ways provided by SAP to exit the standard code and we can write our own code with the help of ABAP workbench. its not new functionality which we are trying to build in sap but its slight enhancement within the same code.
Gap analysis is start point of Realization and once blue print is finished we have to find the realization of sap system for client requirment and there will be certain gaps when compared to system fit.
Those gaps can be closed either by re-engineering of business process to fit with SAP or we have to use USER exits in case of small deviations or complete enhancements with the help of ABAP to fit with the SAP system.