RFC 5199 is currently a gap.
This means, that there is currently RFC with number 5199, but this may not be final.
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 5199 - is currently a gap, there might be a RFC document with number 5199 in the works and might be published.
What to do with a gap document?
There is nothing, you can do at the moment. But as long the gap number is not moved to the Not issued RFC numbers, it might be published in the future. Hence, just check from time to time either the Index of known RFC Gaps, if RFC 5199 is still a gap, or the Index of RFC documents, if RFC 5199 was published meanwhile.