There are a few things to consider if you have a gummy loaf of bread. If the starter is not mature enough, there is not enough yeast and bacteria to proof the dough and maintain the crumb structure as it bakes. Underproofing the dough will lead to gummy, dense sourdough bread with big holes, so you need to be patient and give the dough enough time to proof. Proofing time varies based on the temperature of your kitchen, so do not rely solely on a timer to ensure the dough is ready. If you didn't underproof the loaf, then you might have cut into the loaf too early. Allow the sourdough bread to cool fully before cutting it open.