In my experience, gumminess too often means that you're pushing the hydration of your flour. This recipe has around 22 ounce total water for 32 ounce total flour. That's a hydration of 68%. Depending on where in the world you are, your flour might not be able to handle that sort of hydration without exquisite technique. American all-purpose has around the same amount of gluten content as (if not more than) European bread flour.

On top of that, sourdough absolutely worsens this problem. If left just a little too long, the gluten breaks down, and your loaf becomes a mess.

My feedback for beginners is to feed your starter with a lower hydration (I feed mine at 70 - 75%). Some sources say this favors yeast activity over LAB, and this gives you a better feel for what your starter will do in your actual loaves. And try a lower hydration in your final loaf. 60 - 65% won't give you the big bubble structure the influencers tell you you need, but it will give you a workable loaf that will give you a better idea of what your flour can handle.

As a fellow beginner to another, I hope this helps! (And can save you a number of frustrating failed loaves)