A gap analysis compares two IT architectures in order to identify the difference. This analysis can get quite complex since in the IT world "close" doesn't work, so all differences must be identified.

The "components" that define the system are identified, in both the "Existing IT Architecture" and in the "Target Architecture". If the Existing Architecture is not defined, then it must be defined, at some level of detail, prior to doing a gap analysis. Likewise for the Target Architecture.

If the two architectures differ significantly then it could be difficult to identify and relate components. Identifying components at a high-level initially may help. The bottom line is that differences between the architectures need to be define in enough detailed for the next steps, which look at these difference and other factors to develop or update a Migration Plan.