For the GPU solving the installation of CUDA Driver is necessary, more information here.
To run an optimisation on one or more graphics card(s) the solver priority can be set to GPU solving in the Application Settings - Generative Design Solver. The default solver setting is CPU. GPU solving should be preferred because of increased solving speed with the associated license.
The CPU threads should be defined in a GPU calculation as well because they are used to build up the FE-matrix and the set value is important for CPU as well as GPU computing.
Further information here.