I have found psyllium husk really helps keep gluten free breads and bakes moist for longer. It has similar gelling properties to chia & flaxseed but has the added bonus of retaining moisture & oils.
Some extra info.
Psyllium seed husks, also known as ispaghula, isabgol, or psyllium, are portions of the seeds of the plant Plantago ovata, a native of India and Pakistan. They are hygroscopic, which allows them to expand and become mucilaginous. Psyllium seed husk are indigestible and are a source of soluble dietary fiber. They are used to relieve constipation, irritable bowel syndrome, and diarrhea. They are also used as a regular dietary supplement to improve and maintain regular GI transit. The inert bulk of the husks helps provide a constant volume of solid material irrespective of other aspects of the diet or any disease condition of the gut. Some recent research has shown they may be effective in lowering cholesterol and controlling certain types of diabetes. Other uses include gluten-free baking, where ground psyllium seed husks bind moisture and help make breads less crumbly. The husks are used whole in their natural state, or dried and chopped or powdered for easier consumption. In either of these forms, one takes them by mixing them with water or another fluid. They are also available in capsules.