Fly without GPS and you will be good to go! Just kidding, I've flown with GPS activated during high K-indexes, without issues.
It's been debated whether high K-index is a definite factor to stay on the ground. You should be cautions as usual though in case something unusual happens.
The way I see it is if you are flying for recreation then I would hold out for a better day but if you are trying to earn a living then it maybe worth the chance
I use UAV forecast app and totally relay on it as it has proven to be spot on when I needed it and with my settings set to what I feel comfortable flying with if it says DO NOT FLY I don't better safe than sorry