Why is my sourdough bread dense and gummy?

Usually underproofed. If the dough doesn't jiggle and hold a poke dent, give it more time. Cold kitchens can double proofing time.

Could also be cutting too early - sourdough needs a full 2h cool down or the crumb stays gummy even when perfectly baked.

What's your hydration? Beginners should start around 65-70%, wet dough is much harder to read.

The gummy crumb usually means the inside isn't fully set when you cut into it. Even if your bread looks done on the outside, sourdough really does need that full cooling period - sounds boring but the crumb structure finishes setting as it cools down. Try waiting at least 2 hours before slicing, and if your kitchen's warm you might need even longer. That's probably your main issue if everything else checks out.

Dense and gummy crumb almost always points to moisture that hasn't been driven off, and I've seen this happen plenty of times in my lab work when samples don't reach proper temperature or cool down completely. The thing everyone seems to be zeroing in on is underproofing and cooling time, which are both real issues, but there's something else worth checking: your oven temperature might not be climbing high enough or staying there long enough. If your oven runs cool - and a lot of home ovens do - the interior of the loaf can stay too moist even when the crust looks done. I'd grab an oven thermometer if you don't have one already, since my experience with precision equipment has made me paranoid about temperature readings. You want your actual baking surface hitting at least 475°F consistently, and ideally 500°F for a proper oven spring that creates good crumb structure.

Beyond the oven situation, I'd also look at your dough handling during bulk fermentation. Your starter might be doubling reliably, but that doesn't mean the bulk fermentation is complete. The dough should feel noticeably airier and have visible bubbles throughout when you stretch and fold it toward the end - not just on the surface. If you're shaping at the first sign of rise, the gluten network inside hasn't fully developed, and the crumb stays dense and tight. Give your bulk about 4-5 hours at room temperature (or longer if your kitchen is cool), and honestly at 65-70% hydration like someone mentioned, the dough should be workable enough that you can see and feel the difference between underfermented and ready. Then after baking, let it sit on a wire rack for the full 2+ hours before cutting - I know it's tempting, but I learned the hard way that cutting into warm sourdough just seals in steam and creates that gummy texture no matter what you did right before.