Something like this should work, you will need a contactor or relay to go with it. There is list of relays that is approved with it. I see this is a 1999 dated document, some of those relays are under different names now, but the device with an external CT on the back was mostly what I was trying to point out. Took a little searching to find one, key word that finally yielded results I was looking for was "high amperage" GFCI.
No, not on the LV side of whatever this is.
Cat. No. 6895 is UL-recognized and requires a 120VAC line-side feed for sensor electronics to operate.
And that's the problem here, a GFCI will have an electronic PC board that requires control power. Because the only thing that REQUIRES a GFCI per the NEC is going to be at least a 120V circuit, there is zero incentive for the mfrs to make a PC board that has it's own wide range power supply on board. The only other valid use of a Class A GFCI would be in an industrial control panel built under UL 508A that requires the use of an unevaluated device, in which case you must feed that device through a Class A GFCI and an isolation transformer, and even then, UL shows that you put the GFCI device on the 120V feed ahead of the transformer. But below 30VAC RMS, you do not need the protection of the GFCI at all, you can just use a Class 2 transformer, so there is no reason to make one.
So even if they did make a GFCI that could accept a voltage lower than 120V, it might go down to 48VAC, but never "zero to..." anything. I say 48VAC just because that is the only standard control voltage level in use that is greater than 30VAC RMS but less than 120V. Does that device exist? I doubt it. I looked at the common brands; Leviton, LeGrand/P&S, Hubbell none of them say anything other than 120VAC.
Can someone MAKE one for you? Maybe. Bender makes a wider range of options for GFCI devices, here is one:
http://www.bender-us.com/solutions/gfci.aspx
Buried in the brochure under the available voltage codes (starting at 120VAC by the way) is this statement:
Other voltages available upon request. Contact Bender for more information.
If they need to have it, might be worth a call. But be prepared, you are looking at a $800 device, minimum (last time I had to buy one).