What Snow gorget does for Guillotine:
1) +0.1 ftp to the first WS hit.
2) +10 accuracy to all hits of the WS.
Yes, 100% TP, 200% TP, and 300% TP Guillotine.. all give 0.875 damage multipliers with those amounts of TP.
However, TP != ftp. The sea WS gorgets don't give you .1 TP... they give .1 to your ftp multiplier. In other words, they're not increasing the "TP" of your Guillotine. Guillotine damage does not vary with TP.
Using snow gorget on Guillotine will change Guillotine's original ftp multiplier from 0.875 to 0.975.
0.975/0.875 = ~11.43% increase in damage for the first hit of your WS. (Guillotine here)
And again, the snow gorget also gives +10 accuracy to all hits of your WS.