The SkyClub will be part of the new headhouse, not concourse G. The three existing clubs will be consolidated into one large club.
The new headhouse (and club) is slated for 2021ish, though it looks like everything is already a little behind schedule (shocking!).
Walks at LGA will definitely be getting a whole lot longer, though still not JFK-long. Concourse G will be nothing compared to how far out the new concourse D will be, and some connections look to be treks. Combine the centralized club and longer walks, I don't see the new LGA being quite as relaxed going from club to plane, but this is a small price for a modern facility and better operational capabilities.
Since 739s and a321s have the same wingspan, it looks like every gate will be a321-compatible, at least according to these plans.

source: panynj.gov/about/pdf/Draft-EA-Delta%20Reconfig_LGA_March%202017rev3.pdf
Another view of long walk from headhouse to Concourse D:

(AFAIK, the pictured hotel sticking up and airtrain to nowhere are not yet fully approved).