Sorry, you cannot register the hashtag #gghtcritics as a unique hashtag as it is currently being mentioned on Twitter before.
However, you could still use this hashtag for your event as it may not have been used recently or could be used infrequently, to check try searching Twitter.
As another individual or company is already using #gghtcritics, you should try giving it a little tweak. You could try adding a relevant word or date and keep checking. Once you hit the the jackpot and have the perfect start to a great campaign, we'll let you know.