If I am busy and do not look at the forum for a day , I do not see a way to catch up. Maybe there is a way to do this that I have not noticed
There's a lot of redundancy built into GJ, and multiple ways to catch up depending on user priorities and preferences, so grain of salt and all that....
First thing I do is go to Watched (the new Subscriptions). That shows me all new activity in only the threads I am Watching. I could use Preferences to use Alerts (the bell top right) to show me the same thing, but I find scrolling through my Watched lists to be handier.
To see if there are any interesting New Threads, I like to do that by going only to my favorite forums. I can scan a page or two and see all the new threads there.
For me, a running list, in descending chronological order, of ALL new posts, from ALL forums, including those I have no interest in, is too broad and undiscriminating. I see that you can apply Filters to New Posts so you can eliminate All Unread, and just see new posts if it's in Watched content, Threads in which you've participated, and Threads you've started, but that would essentially just be redundant to my first step, filtering everything but my Watched threads in the New Posts window, and again, they are not organized there by forum, they are organized only by time.
With the launch of the new GUI, I've actually been much more intrigued by user habits than I was before. Probably wear off again once everyone settles in.