Gretchen is the woman in the flashbacks in episode 3, when Walt was at the chalk board in grad school, talking about the composition of the human body. Presumably they were dating before he met Skyler, but the show didn't mention how far apart the relationships were, or if there was any overlap (could have been dramatic...?)
"That's how [the episode] ends??!!" is the alternate title of the show, haha. Lots of cliffhangers that are simultaneously amazing and frustrating at the same time. They really crank it up to 11, and then they find a few more scales after that. You'll swear it's gotta be a season finale, and you're just 3 episodes in.
You said you're not looking forward to the cancer side of the story, but I'm confident you'll find it rewarding how they do it. These writers aren't just world class, they're legendary. 20 nominations for writing by the Writers Guild of America alone, plus some Oscars and an endless list of accolades on Wikipedia.