Bad Speculation - How do you get DRK from GLA?

[Steady]

Well there was the fact that when jobs were originally introduced Yoshi-P explicitly stated something to the effect of they were thinking of making DRK a job from GLA.

Also, both are (or have been) sword-wielding magic users.

There's also nothing stopping them retconning GLA's skillset.

Sure, they could... or they could just use Marauder's and not have to change much of anything.