The two tests are independent of each other. you can register for anyone of those two tests at any point in time.... The results are not linked/connected in any way.
Some folks take the GMAT first (thinking it is a harder test and they should get it out of the way first and TOEFL will be easier).
Others take TOEFL first and a third group takes them both around the same time.
I took my TOEFL 2 days before the GMAT at the same test center I took my GMAT later on. Helped me to get a bit more familiar with the test center since TOEFL is quite a bit easier from the stress perspective (my opinion that is)
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