Clebsch-Gordan and the theta filtration for modular representations of ${\mathrm{G}\mathrm{L}}_2({\mathbb{F}}_q)$

Let $p$ be a prime. We solve two problems in the mod $p$ representation theory of ${\mathrm{G}\mathrm{L}}_2({\mathbb{F}}_q)$ where $q=p^f$. We first prove a Clebsch-Gordan decomposition theorem for the tensor product of two mod $p$ representations of ${\mathrm{G}\mathrm{L}}_2({\mathbb{F}}_q)$. As an application, we use this to guess the structure of quotients of symmetric power representations of ${\mathrm{G}\mathrm{L}}_2({\mathbb{F}}_q)$ by submodules in the theta filtration. We then give a direct proof of this structure showing that such quotients are built out of principal series representations.