c# entity-framework sql sql-server

Performance impact of using GUID in SQL Server

I have tried searching this before asking but every result I have found mentions GUIDs as a PK which is not the case here.

I have a database that’s using INT as the PK on all tables. However the data is accessed via API calls and a requirement was for the INT value not to be returned or used in any API. Therefore I thought of having an extra column on the tables containing a GUID.

Now my question is, if I Index the GUID column what kind of performance impact will this have? Would it be positive or negative? Bear in mind the GUID is NOT a PK or FK.