The chromatic gamut is the property of output device, not a recording device.

The colorimetric input device is characterised with it's compliance to Maxwell-Ives criterion - i.e. how metamerically close it is to the CIE standard observer - the equivalent of photopic human vision. This compliance can be roughly characterised with one value, the example of it is DxOMark's metamerism score and metamerism is the most important property of colour recording device which should be characterised.

Theoretically, a recording device CAN have limited input chromatic gamut if, for example, it uses colour matching functions with negative responses present (I am just making it up because there is no practical possibility of doing this, a device doing this would be a broken device):

If it happens so that electronics of that imaginary device are not capable of producing negative image data then wavelengths from 400 to 600 would be out of gamut for such device while some mixtures of wavelengths will be inside of gamut.

However, there is no obligation to use those functions with negative parts because there are completely positive colour matching functions which give accurate metamerism as well because when you have full chromatic data at all image points you can use arithmetic operations on all channels to deduce true colour:

No cameras I've seen have actual negative responses.

The most important thing - metamerism - gets broken in all consumer cameras regardless. You can't deduce true colours from the image data which a usual camera records. Here's an example of trying to deduce true colour from Nikon D70 data taken from http://theory.uchicago.edu/:

This graph shows how well colours can be reproduced. Knowing that a CIE XYZ is a space of imaginary super-saturated colours you can see that colour reproduction accuracy is a trainwreck. And to top it off D70 image data gets clipped when transformed to XYZ space - which is in a sense the gamut limitation because XYZ is the widest colour space used after RAW processing.

So, there is no such thing as input gamut in the sense you are asking about.