Doing some off-season gear maintenance and resealing my Gore-tex duck jacket is close to the top of the list. A quick search indicates Nikwax spray is the go to solution for both effectiveness and cost. good amazon reviews as well. Anyone with experience on this product or other recommendations?

Haven't used the spray, just the wash. It restored the waterproofing in my Gore-Tex jacket that had completely lost all waterproofing . I now use it on all my DWR rain jackets to keep them from getting to that point. Does its job. I don't clean them any other way.

2 things to consider:
if the gore-tex membrane is bad the jacket's done for, it'll never really be waterproof again
if you stay dry in the jacket, but the outer fabric gets soaked (water doesn't bead off) then the durable water repellent (dwr) treatment is broken down. Usually a ride in the dryer will fix this, as it melts and redistributes the molecules or particles of the treatment throughout the fibers, and water will bead off your jacket again. A ride in the washer can also help a waterproof/breathable membrane that isn't working so great, as grease and funk can mess it up.
I don't know what the best stuff on the market is these days but the nikwax tx-direct wash-in stuff used to be the go-to. I think the nikwax spray will make your breathable jacket non-breathable

Great pointer. Thanks

It may be a situation where the goretex membrane is done for. I'll give the heat treatment a shot, then supplement with a spray to see where it gets me. However, rain will permeate the outer layer and the inner jacket liner as well, so the goretex may in fact be done. Thanks.