There are so many interpretations, and we have so little solid fact, you can't make solid arguments about any of this.
One site breaks down Norse magic very distinctly:
seiđr is the magic of women, involving witchcraft, prophecy, channelling, illusion, and mind-control, and it is the magic of runes;
spá is the magic of seers (usually women), a high station, who can read the form of fate and the order of the universe, and its practitioners include the Norns and the völva of the Eddas; and
galdr is the singing magic of men.
By this breakdown,
GURPS Vikings only covers seiđr, unless you want to consider good mystics to be employing spá.