I'd like to think I'm usually a fairly realistic person when it comes to the performance of the GOP in the South, and what chances both they and Democrats have in various places.
With that being said, I see no viable, realistic path for the GOP to hold onto this seat. Gwinnett is one of those places where even somebody like me can turn their head for a few months and already be out of touch with how much demographic shift and growth has occurred in a broader area. It's changing faster than SoCal. It's changing faster than NoVA. I'm not sure there's even a place anywhere in the country that is comparable to it in this regard (maybe another area within the broader metro).
Democrats won't need it to be a good year for them to win. Hell, the GOP could probably win the House PV and this district would still have a decent chance of flipping. Besides the huge rate of demographic turnover and growth, this is also ground zero in GA for Latino and Asian voters (who, while turning out in big numbers in 2018 like everybody else, still lagged the electorate as a whole - their turnout will be higher in 2020, with or without Abrams).
Just because I consider you a fairly neutral and informed observer of Georgia politics, I'll ask: how would you rate Woodall's 2018 campaign operation? A lot of GA Republicans didn't think he put forth much effort and I think that's a major reason why Woodall is retiring.
As for GA-07, yes it's an increasingly diverse district but let's not conflate Gwinnett County trends with GA-07 trends. The most Democratic parts of Gwinnett aren't even in GA-07, and Buford and Sugar Hill are still rabid pro-Trump exurbia that will be turning out for him big time in 2020.