I thought it was somewhere along that line, too. Something in Walt's impulsivity that made him storm out of the relationship and the company, and him stewing on that resentment ever since. I'm glad the Grey Matter story came back around, but we'll have to live with not knowing exactly what split them up originally.

Yes! I feel like Walt's entire arc hinges on what happened between he and Gretchen and why he inexplicably (so far) left the company. If we don't find that out I will be so sad.