If we're talking about Windows Perfmon, it can generate output in its own proprietary binary format, CSV, or store/dump in a SQL database. You could just set up Perfmon to log results as CSV or in database, then parse/extract the Perfmon data in CSV/database to use by Gatling reporting.
For that part, I generally use the command line versions of perfmon rather than the GUI to export/convert perfmon log formats as well as start/stop/log on demand like with typeperf.exe etc.