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You shouldn’t phrase your own licence and hope it will be considered open-source and GPL-compatible as there are plenty of open-source licences to choose from. Creating your own licence may put your users and you under risk of abuse and will contribute to the problem of licence proliferation, and may confuse your users who will need to read your unusual licence in order to study it.
Using your own custom licence deprives you of the benefit of using a well-reviewed, well-understood and well-debugged licence. This is similar to writing your own program from scratch instead of using an existing, well-tested software solution that already provides that functionality. (Thanks to Omer Zak for this analogy.)
So, don’t do that.