Gluten free baked good coming out bitter
Both of those ingredients are not bitter unless they are rancid. You may be using too much baking soda?

A basic gluten free flour is usually a combination of white rice flour, brown rice flour, sweet rice flour, tapioca starch and xanthan or guar gum. From there it can go into buckwheat, bean flours, quinoa, etc. The gums give resilience and elasticity to the dough (along with the tapioca and sweet rice flour) and replace the action of the gluten which creates a structure for baked goods to rise.

There's lots of gluten free flour recipes that come up with a Google search. It's sometimes easier to start with those than trying to create one from scratch. 

Oats BTW, are not strictly gluten free because of cross contamination when growing or processing. The package should state that they are gluten free.