The g-code is the g-code and can't override the max speed of your machine. That is done under motor tuning in Deskcnc.
Under one of the tabs at the top of the screen is a sub-tab called setup, I think. There you set the acceleration profile and the max step rate. It is not too difficult to do and once you get the hang of t you can set up a new machine in an hour.
If you have any more questions please ask.