Your current model has no way of generating a trend. You will have to write a model that can do that if that’s your goal. The simplest case is a model whose mean changes linearly with time. Perhaps this notebook can be helpful: GLM: Linear regression — PyMC3 3.10.0 documentation
The model in the example uses a Normal distribution but the same logic can be applied to a Gamma distribution. There is a snippet example from another user in this thread: Gamma GLM for epidemic growth model