Last time I went up Gasgale gill (before the recent flooding though) we followed the low path right alongside the beck (beck would be on your right going up) I can only imagine that route will be a mess now. Best bet is as soon as you cross the wee bridge at the start, head about 50m or so (approx.!) up the steep path ahead, and that takes you onto a slightly higher path that eventually meets up with the lower one farther up.
Let me know what way you end up going ir, planning a weekend there soon, and want to find the funnest way from Seatoller over either Causey/grizedale/high spy to get to Keswick, and then finish on walla crags and Watendlath descent.