If the hydration is too high for your dough it can lead to an overly wet or gummy interior. If the dough feels "soupy" or very "slack" then try pulling back the water percentage by 5% and see if that helps.

Make sure to bake your loaves completely. The interior should register around 204-206°F (95-96°C) or higher.

If your loaf is under proofed then this will typically lead to a gummy or "wet" textured interior. Make sure your starter and levain are very vigorous and strong when you use them. This is very important! From there, make sure to bulk ferment your dough fully (use the images you see in my posts to guide you on what the dough should look/feel like). You want the dough to be alive and aerated before you divide and shape.

From there, a full and complete proof is also very important.

A few ideas there for you, let me know how the next attempt goes!

i set my gas oven to 500° and heated everything up for an hour, followed all the steps. I took the lava rocks out after 20 minutes and the internal temperature was 210° after another 15 minutes of baking.

Buuuut it's gummy. It's got a great flavor but gummy, lol.

I may need to invest in a real baking steel. I used an upside cookie sheet, could that have made the crumb gummy?

That one isn't needed, these days, I don't use it!

That works well, for sure!

Ha ha, definitely avoid the asbestos. Fire or refractory bricks will also work very well!

Thick refractory brick or stones will also work very well!