The Toffees have triggered the midfielder's £7.1million release clause and he has been discussing terms with them this week
Gana's clause expires at the end of the transfer window but only Everton have made an official bid
Villa, relegated from the Premier League last season, want to keep him but Di Matteo is resigned to seeing the Senegal international leave after just one season at the club.
He said: "It's public Everton have triggered his clause and he's going to leave
There's no news so far but the expectation is, once personal terms are agreed and a medical (is done), he's going to go
"There are other clubs interested, that's right, but the only official bid so far has been from Everton
"There's not much I can do because he had a clause in his contract and that's something I inherited and my hands are tied
We will be losing him
"The fact he is leaving is basically down to the clause he has in his contract, otherwise there would be no chance
"We're not a club looking to sell, we don't have to, we don't need to."
Gana joined Villa from Lille last summer and made 38 appearances, scoring once, as the club were relegated from the top flight last season - winning just three league games
He could become Everton's first major signing of the summer after they appointed former Leicester assistant manager Steve Walsh, who helped find Riyad Mahrez and N'Golo Kante, as director of football.
New boss Ronald Koeman has signed Maarten Stekelenburg from Fulham but is yet to make a serious move in the transfer market.
Jordan Veretout, Jordan Amavi and Jordan Aye w have also been linked with moves from Villa while Brad Guzan joined Middlesbrough on Friday - and could face his former club when Boro travel to Villa Park for a friendly on Saturday
Source : PA
Source: PA