Complexity (SolverMaxMemory) defines the maximum available memory for the stiffness matrix.
It corresponds to the resolution of the model and thus determines the calculation time.
Increasing this values increases the resolution and thus calculation time!
It does not define the exact amount of memory the optimization uses, but the memory that is needed in the worst case if no material can be reduced due to the optimization setup.
You can either choose a percentage of your available RAM or an exact value.
With GPU Solving, the GPU RAM must not be exceeded (most Graphics Card Boards feature 1 GB to 14 GB. If necessary, CPU can be used to solve with even more than 100 GB).
Further information here.