Because gamin's IUSE doesn't list kernel_linux, nor is it in use.(local.)desc, nor is it enabled in the default profile, most users probably won't know about it, and everybody is installing gamin without inotify support.
This is NOT something that users should ever modify. Portage shouldn't moan about USE_EXPANDed stuff and preferably shouldn't even display userland/kernel "flags" in emerge --info (my personal view though). Those are based on your userland (as well as elibc_glibc etc.).
Not a gamin bug, there's nothing wrong w/ the ebuild
*** This bug has been marked as a duplicate of 70648 ***
Ah! I didn't know about the USE_EXPAND issue. Makes sense now - no need to set this USE flag explicitly.