Im on my second conjurer now; both times I've chosen Gimbles at 10 and am convinced it's the way to go. But my first alt I took Ganlas at 20 and well, it sucked. Cast time was long and damage was mediocre. I was so excited to get it, after being so impressed by Gimble and then was immediately dissapointed when I went out and used it. I dropped it from my hotbar and just kept Gimble, even thought it was greyed out.
In rare circumstances, Ganla will come in handy. But it's dissapointing as hell in comparison to Gimble.