In Chicago, WLS-TV has had a successful afternoon lineup for years that they're not gonna change it so easily. General Hospital is currently on at 2pm, Inside Edition at 3pm, Jeopardy at 3:30pm, local news from 4pm - 5:30 pm, World News Tonight at 5:30pm, local news at 6pm, & Wheel of Fortune at 6:30pm. If GMA: Afternoon will be a network show, I could see this show pre-empted by many affiliates (possibly by O&O WLS-TV, unless it airs in The Revolution's timeslot), because the affiliates usually have successful programs in the afternoon timeslots.
For affiliates & some O&O ABC stations that now have news in Oprah's old timeslot (WLS-TV was the exception, as they had her on at 9am), it'll be a hard sell for GMA: Afternoon, as the news might be doing that well that they're not giving it up. I can't blame those stations, or even the viewers who would like to have a 4pm newscast.