Gummy Bread...

I find a preheated baking stone (I use a cast iron griddle) gives the loaf a kick up the backside and seems to stop the crumb being dense at the bottom.

What are the fermentation times, how long are you kneading etc?

My gut reaction is that it is not baked through. Unfortunately, ovens can be very convincing liars: it says that it is 375, but it may be running a bit cold; or, it is not maintaining itself properly--many ovens fluctuate because they vent and reheat, vent and reheat, vent and reheat, in order to try to stay at a constant temperature. However, it's a pretty inefficient system that doesn't always work terribly well.

Were I you, I would crank up the oven temperature to at least 425 to start the bake, turning it down after 10 or 12 minutes to 375 or 400. The crust will probably come out a little darker, but you have a better chance of reducing the gumminess. Home ovens are also insistent bargainers: a willingness to compromise is always necessary.

Good luck !

Unless I've missed it, there's nothing in what you've said nor in any other reply that asks the internal temperature of the bread when you remove it from the oven.  If you don't measure the internal temperature of your breads, try that using a fairly inexpensive cooking thermometer easily purchased at your local hardware store.   I use one with a pretty thin probe. To avoid poking a hole in the top of a loaf, I remove it from the oven, turn it over, and poke the probe through the bottom crust.  For the breads you're making, internal temperature should hover around 200 degrees, but certainly be at or above 195 degrees.  Above 205 degrees is probably too high.

If you do this, please report what you find.

Get a new thermometer.  Plunge the thermometer from the middle of the top down to the middle of the loaf.  The temperature should be 195 to 205°F.  The small hole made in the middle of the loaf should affect only one slice. If you bake several loaves at the same time, then you need to test only the one in the middle.

Ford

to 500 F and bake the bread at 450 F once it hits the oven.  If you don't have a stone than put two jelly roll pans together, turn them over and bake on them.  Use 2 of Sylvia's steaming pans for steam ( a pan half full of water with a kitchen towel rolled up in them.)  Put them on the bottom rack when the oven hits 500 F and leave them for 15 minutes for the steam to start billowing, before putting the bread in.  Don't put your face near the door when you open it.  Steam for 15 minutes then take the steam out and continue baking, if you don't have convection, until an instant read thermometer reads 205 F on the inside.  If you have convection turn the oven down to 425 F when the steam comes out and turn on the fan

No more gummy crumb will be the result.

Happy Baking

I do agree with the comments on temperature - 375F does seem a bit cool for a basic white bread. I tend to start it at about 270C (520F), turn down to about 220C (430F) for the first 20min, then out the pan and finish at 180C (350F) for about another 20-30min. I know not all ovens will get up that high, but the point is that hotter is often better. Bread doesn't readily burn.

I certainly wouldn't turn it down when it spikes. There's no such thing as too hot an oven when baking bread, but too cool an oven can be a bad thing. Let it spike. The bread will be fine.

I will add that when I was having trouble with gummy crumb, it was not to do with underbaking. It didn't seem to matter how long I baked them for, and even if they hit the right internal temperature they would still be gummy. Sometimes. Not all the time (which makes it almost more annoying). However, those were whole wheat loaves and loaves with oats in them, so the principles at work are probably different.