A counterintuitive and odd term used by some elderly IT folks as opposite/dual to System of Record.
You can copy data in there but you shall have a copy of the original data with you.
Essentially is a way to avoid full blown responsibilities and requirements of a System of Record,
and cherry pick only requirements that are easy to implement.
This definition also is wobbly and specific for every single data object of the information model supported in the system.
Some people interpret it as lifecycle information is stored also else where, some people interpret this as the full information is stored also elsewhere.
Some people leave this poor man's details as "undefined behaviour" so that is possible to go through architecture approvals and it is then possible to resell and recast the system into something it was not fit for purpose/fit for use at the beginning.
GAP is a System of Copy
We are a System of Copy we are not responsible of Disaster Recovery, in fact you have a copy
We are a System of Copy we are not responsible for Lifecycle, in fact the System of Record is.
We are a System of Copy we are not responsible for GDPR regulations in fact you are.