Gelatin is a protein and as it degrades starts to liquefy.  You might be able to use a spindle viscometer.

Alternatively, you might be able to count the microbes that are responsible for biodegradation.

A SDS-PAGE gel would give a good indication of the molecular weight of the gelatin before and after the degradation. However, gelatin is found in different grades, so I would run one sample of the gelatin before any treatment to find out the initial distribution of weights found in your sample.

Gelatin is a mixture of many polypeptides (over 1 million MW).  You will find as the temperature increases, the water content increases, and time increases, these polypeptides breakdown into smaller molecular weights.  Thus, SEC has been a mainstay but it does not tell you much about bio-degradation.