It appears Heyward wants and wanted more money than he's worth. So, getting Miller+ makes sense.
Upton is also a FA and his brother was going to be moved at all costs. He also has proved that he's a very good bat in stretches but isn't a franchise cornerstone because he disappears for a month at a time.
Gattis could wear an oven mitten out in the field and be no worse a fielder. If he wasn't a catcher (and he wasn't) then he had to be a DH long term. My problem with his trade is that the Astros had a lot of bats which could have come back to the Braves that didn't because of the Rainman like pitching, pitching, pitching mantra.
All should have been moved. The return didn't include enough bats considering the condition of the offseason FA market.