Tricky! I'd really love to see a little Gillian - it's a great name that needs reviving - but Joanna is my mother's name & I like that a lot too. Evelyn is nice enough but not a fave. Would you ever shorten Joanna or Gillian?
I would shorten Joanna to Jo or Joey on occasion, but I wouldn't shorten Gillian. I don't like Gill/Jill or Jilly, though Gilly pronounced with a hard G isn't so bad. Very GoT.