I've been planning on doing a nut swap on a bass to convert to CGCF and gauge was one of the first and only questions.
From what I've found, and know, if you like thicker strings and won't be swapping between D standard and drop C, and will be staying at C,
.125, .95, .75, .60
It's a custom set and pretty much what someone above said you could do.
My personal, general, rule is to move .005 up with every half step down, and vice versa to maintain feel (in string tension).
The G, C and F strings are .010 up from a standard set because they are a step down (minus the F; personal preference) and the C is .20 down from a standard 105 E. (115 D, 105 E)
I find a .115 D is very good in feel.
If you want to switch between C and D, .120 is probably the thickest you can go and you may need to settle for a .115 because D is 1.5 steps up from the designed tuning (B) while C is only a half step.
If you like lighter strings, and still want to keep tension, the Drop Down .50-115(?) DR set would probably suit you.
Just keep in mind the strings may feel loose with this set, and also, you run NO risk of damaging your bass, though it may not work as well as it could until it is set up for the gauge and tuning it is in. You may need the nut filed to fit thicker strings. Experiment with the G, C, and F strings first, then start swapping out Low C's once your happy to see what you like best.