Display Screen or Portion Thereof with a Transitional Graphical User Interface for a Gaming System

FIG. 1 is a front view of a first image in a sequence of a first embodiment of a display screen or portion thereof with a transitional graphical user interface for gaming system showing my new design;

FIG. 2 is a front view of a second image thereof;

FIG. 3 is a front view of a third image thereof;

FIG. 4 is a front view of a first image in a sequence of a second embodiment of a display screen or portion thereof with a transitional graphical user interface for gaming system showing my new design;

FIG. 5 is a front view of a second image thereof;

FIG. 6 is a front view of a third image thereof;

FIG. 7 is a front view of a first image in a sequence of a third embodiment of a display screen or portion thereof with a transitional graphical user interface for gaming system showing my new design;

FIG. 8 is a front view of a second image thereof; and,

FIG. 9 is a front view of a third image thereof.

The outer broken line rectangle shows the display screen or portion thereof and forms no part of the claimed design. The broken lines illustrating a finger show environment only, form no part of the claimed design. All other broken lines show portions of the graphical user interface and form no part of the claimed design.