I noticed that when I run the solver via the GAMS IDE or Python API I don't use more than ~33% of my RAM. Is there an artificial barrier which I can put up or disable?
As @ErwinKalvelagen pointed out: by default gams cplex uses only 1 thread which results in a low usage of the pc ressources.
In order to change this one has to increase the thread number so that multiple cores can be used at the same time: https://support.gams.com/solver:multiple_cplex_threads
This analogy might help: CPU usage is like the power output of an engine - more is better in terms of performance. Memory usage is more like the heat produced by the engine - too much heat, aka memory capacity exceeded, and the engine breaks down.
There is simply no reason in trying to increase or even max out memory usage.