Most congregations have somebody on the Bethel e-mail grapevine. News like that spreads quickly, but informally.
The Watchtower is prepared many months in advance, the "June 1" issue was actually prepared in February or March or so. There will be a brief one-page article on Barber's death in a month or 2.
There are no automatic replacements for GB members - there is no fixed number that has to be filled. However, it is quite likely that 1 or 2 new members will be selected at some point. Perhaps they will wait for Barr to keel over & name 2 at once - that seems to be the way they've done it lately.
I'd bet a nickel that the more ambitious of the "Nethinim" are jumping all over themselves, trying to get noticed so they can be named to the GB.