Fishing on the Gallatin is really good right now as long as the weather isn't too nice and either the West fork or Taylor's fork doesn't muddy up due to the melting snow in the creek bottoms. Now is traditionally one of the truly great times to fish the Gallatin in relative peace and quiet. No summer boaters and rafters to contend with. Fish are definitely in the deep off speed pools.
flows:299 cfs
hatches: midges
patterns:
nymphs: 16-20 lightning bugs, small pts, copper johns, disco midges, hare's ears are good choices behind san juans in size 10 as well as small stonefly patterns.
dries: again, if you can find the fish up, it'll be on midges. Small (18-20) hi-vis para adams, royal wulffs, and H&L variants are good choices.