I got one of those the other day as well. This is only for the purchased gas cost (the PGC line item on your bill) – it does not affect the distribution cost, which is at least half of my monthly bill. For me it would only have knocked an average of $2.75 off my monthly bill for the last year.
Also, the fine print is worrisome to me. It looks like after 2020 they can either offer another fixed term or switch to monthly charging with an additional adder of up to $0.600/Therm, so it could really jack your rate up.