Actually I used to think gummy texture meant under-baked, but cutting warm bread causes it too. @masontorres might be onto something with over-proofing.
Wait you used to cut bread while it was still warm? That's like bread murder right there. The steam needs to escape or you get a soggy mess. I learned that the hard way with a whole loaf of sourdough. It turned into a dense brick because I couldn't wait for it to cool.