The gnumeric ebuild does not list nasm as a dependency, because technically gnumeric does not have a software dependency upon nasm. It does, however, have a build dependency; nasm is required in order to compile gnumeric. Therefore, on a new Gentoo installation that does not yet have nasm, 'emerge gnumeric' will fail until the user performs a manual 'emerge nasm'.
I don't see where gnumeric requires anything from the nasm package and you offer no error messages etc..Please reopen if you have further information.
To reproduce this, I unmerged nasm, since it was not present in my Gentoo installation before first attempting to emerge gnumeric. I then emerged gnumeric again, and it completed. This means that I apparently didn't read carefully enough, and it was one of the dependencies to Gnumeric that requires nasm to build but doesn't list it as a dependency. Unfortunately, I don't know which one, and since emerge -C does not remove dependencies, at my current state of Gentoo knowledge this means I can reproduce the problem only by burning down my Gentoo install and starting over in the same way. Naturally, I don't really want to do this. If anyone tracking this bug is bringing up a new Gentoo install, please try emerging gnumeric on a system that does not yet have either gnome or nasm built, to try to get in the entire dependency tree and see where it breaks.