Using 100 % oat flour will usually result in gummy texture (though it depends on personal taste really), so if you wanted to follow Ella's recipe and it be less gummy then you could add some almond flour and tapioca (and decrease the oat flour!). (deliciouslyella.com)
For some reason, I keep getting a really gummy texture with this flour. (fakefoodfree.com)
I find sweet rice flour adds stickiness, working as a binder that also tends to increase the chewy texture, almost leading toward gummy is used in large quantities, tapioca starch seems to work as a binder adding chewiness without the same gummy texture, leaving baked goods a little more delicate and tender.. (snixykitchen.com)