The main gatcombe House is home to Princess Anne and Tim. It was reported, but never confirmed I don' think, a while back that Zara and Mike had moved into Aston Farm, a 7 bedroom farm house and estate next door to Gatcombe that was purchased by the Queen and the use of which was given to Mark Phillips when he and Anne divorced (its been reported in a number of places that ownership was not transferred to Mark but that it was put in trust for Zara and Peter to ensure it stayed in the family) They had previously lived in "The Bothy" a smaller house on the main estate where they lived before moving into Cheltenham.
GIRL ABOUT TOWN: Dad's old farmhouse? Bling it on, says Zara | Daily Mail Online
This would suggest to me that Peter may well be the one who will eventually take over Gatcombe itself, he helps run the annual Festival of Eventing that takes place there annually.
There has been a lot of planning applications in recent years, especially around Aston Farm including creating 3 /4 bedroom houses. I suspect Peter lives in one of these as does Autumn (if she does still indeed live on the estate which I'm sure she does for the girls at least)
Unlike a lot of other royal residences it is not owned by the Crown Estate and leased but was instead purchased by the Queen herself as a gift to Anne on her marriage. The estate runs at around 700 acres an includes, not surprisingly, a vast amount of stabling facilities.