Gravity Movement: A character attempting Acrobatics, Athletics, or Close Combat tests while in non-standard gravity reduces their Expertise and Focus ranks to those of their Extraplanetary skill. Moving easily in space or on unterraformed worlds is challenging even for the most physically adept.
The Extraplanetary Skill itself isn’t used, its Exp/Foc just defines a max cap foo using the other skills when i a high/low/zero gravity environment, all the skills are available and all various actions are possible assuming GM circumstance allows. (Yes, thats about the same as just using the Extraplanetary skill, but its important for talents and actions you can take etc, also you may not have some skills above the cap while others are.)
The part about modifying difficulty levels refers mostly to doing things like hauling around a heavy weight in ZeeGee (lower difficulty by 1) or stopping your momentum in ZeeGee when heading towards a death trap while towing a heavy weight (increase difficulty by 1, momentum is a PITA). Or maybe manipulating fine objects through bulky space suit gloves and other such incidental challenges.
For standard everyday actions the use of Extraplanetary capping ballistics/Acrobatics/Melee etc should be enough of a penalty, just use Complications themed to the gravity to illustrate that even a skilled individual can sometimes not deal with variable gravity well.