Given that he shares writing credits on Infallible and Pendulum, I think Stone sells himself short by calling Let the Records Play his "main contribution in terms of songwriting."
Stereogum published a great interview with Stone Gossard today highlighting his contributions to Lightning Bolt, and they sum him up nicely.
There’s a good chance Stone Gossard doesn’t entirely get the credit he deserves. He’s probably best known for his role as rhythm guitarist of Pearl Jam, but such a distinction overlooks the length and complexity of the guy’s resume. It dates back much further than Pearl Jam, to Mother Love Bone, and to Green River before that, a band sometimes credited with inventing grunge. Even within the context of Pearl Jam itself, with such a visible frontman as Eddie Vedder, it’s all too easy to underestimate Gossard’s influence.Stone also talks a little about his writing technique.
I do a lot of writing at home with the guitar and that’s just when you get fifteen minutes or everybody leaves the house or you have too many cups of coffee and you just go up to your little room and you play for a while. I’ll record to my iPhone, and after I get a handful of those I have a studio that I’m able to use and I’ll book time with my favorite engineer. I can go in there and re-demo the demos to get them at least up to speed, get them recorded someplace other than just my phone. You flesh them out a little bit, add some instrumentation to them, and see what they can be.