Well I'm a rock climber, snowboarder, skateboarder and a dirtbiker so I'm pretty nimble and I have excellent balance. Also at 6'2" and 167 I'm pretty skinny so I fit through the slider. If the tailgate is down I just jump up, land with the edge of my foot on the gate and lean in. If you move quick enough it is easy to keep balance that way. If the gate is up I just get up on the step bumper, crawl over the tailgate and grab the hatch down. I've been known to go in through the window from the front seat and crawl across the shelving too.
The girls usually leave the gate up and then step over the gate. In reality whatever works is what works. Most people find it easier to get in and out with the gate up.