A guitar set has 6 strings, from high to low.
For my special gCeA tuning, use these:
1. E string (high) / 2. B string: use one of these in the ukulele 2nd string position, and tune to E above middle C. You will need to experiment to see which works better. I would start with the E string, since the worst that can happen is it's too loose/floppy and you decide you need the thicker B string.
3. G string - use in the ukulele 1st string position, and tune to A below middle C
4. D string (wound) - use in the ukulele 4th string position, and tune to G below middle C
5. A string (wound) - use in the ukulele 3rd string position, and tune to C an octave below middle C
6. E string (wound, lowest) - do not use.