Gerry Mulligan: ''Little Big Horn.'' (GRP Digital GRP-A-1003) Baritone saxophonist Gerry Mulligan, with the release of ''Little Big Horn'' has taken his first stride forward (or backward, depending on the point of view) into the world of pop music. This is a slick package with very little interest for jazz listeners, especially those who have admired Mulligan's work over the years. Most of the selections range from bland to banal, and the tired pop-rock backgrounds hardly provide a suitable setting for Mulligan improvisations. Even the compositions aren't up to Mulligan's usual standard. What a pity that an album like this should follow in the footsteps of his Grammy-winning big-band album ''Walk on the Water.'' There are some pretty melodies and some good solos by Mulligan, but nothing can change the fact that context is of vital importance and is bound to affect the over-all feel of a musical performance. Mulligan sings nicely enough on his tune, ''I Never Was a Young Man,'' but it's really a forgettable song. Only ''Sun on the Stairs,'' another Mulligan composition, approaches the quality of former Mulligan efforts, but even this tune is marred by the corny feel of the rhythm section. Arrangements are by Mulligan and Dave Grusin, who also plays piano and electric piano on the album.