At the moment there is no NURBS programming library in portage (unless I have overlooked it). sci-libs/nurbs++ from sunrise has not been updated since 2002 and so does not compile.
SISL sounds like a really professional NURBS library that is under continual development.
Sorry, I should clarify and correct myself that although there has been recent work to address the compilation errors of nurbs++, it appears that development of the library stalled in 2002 (and it still does not compile for me).
Anyway... that is kind of beside the point because even if nurbs++ was working, I still think that SISL is a worthy addition to Gentoo in its own right.