The feature matrix for the two amplifier options is shown below.
The GT has a preamplifier section with input gain control, in addition to some noise reduction features that are not as critical on the newer Spike and JJP titles (which are not as susceptible to noise originating from traditional power sources, etc.).
The KO has the same power rating, level and tone controls as the GT but eliminates the preamplifier section.
The KO is offered at a $45 lower price point at this time.
The GT is not offered on Rush because of the dynamic complexity of the music track and its auto-muting feature. The GT remains a great choice for other Spike and JJP titles if you find value in the additional features.
Both options provide a means to run an external sub on Spike.
Hope that helps, and thanks for asking!