RFC 575 is not issued.
This means, that there is no RFC with number 575, and there never will be.
There are three different publishing-states of RFC-documents:
What is the main difference between "gap" and "not issued"?
A "not issued"-RFC-document is not available at the moment and there never will be a document with this number. It might be, that the document was never finished or that the document was already obsolete before it had been published and publishing was skipped. But its number was already taken and can not be reused. Note, if a RFC document is issued, it remains so. It might be, that the document is updated or even obsoleted by a later one. Nevertheless the issued document remains, just its document-state might change.
A "gap"-RFC-document is not available at the moment. It might be, that the document is dropped, but decision has not been made officially, or that the document is just not yet completed or published. That means, even if - say RFC 575 - is currently a gap, there might be a RFC document with number 575 in the works and might be published.
What to do with a gap document?
There is nothing, you can do. If the number is stated "not issued", there never will be a RFC document with this number. See the Index of RFC documents for published documents.