From Sam Hartman:

A section of the GIST document be added clearly indicating when a GIST
implementation can modify a packet not targeted for itself and when it is
expected to intercept such packets.  That is currently scattered too much
throughout the document.  Specific care should be given to cases where a GIST
node will not forward traffic for which it would normally act as a router.  In
particular, I think it would be harmful if traffic that happened to be on the
GIST UDP port were discarded if it clearly had no relation to GIST.