Hamsterley (and I say that as someone who lives 25 minutes from Gisburn).
Do the black, then head over to the start of K-Line, do those sections, then climb up to Section 13 and finish from there.
The back end of the official red is very boring.
Which diversions are in place at Gisburn?
The boardwalk section up from the quarry at the start of Hope Line was shut, but I did see pictures of people levelling some of the climbing bits, so they may have finished by now.
As an afterthought, if you’re staying near Kielder, you are the same driving time from Keswick as you are from Hamsterley, and the Lakes is a lot nearer than Gisburn.