The Toulon fly-half, 33, retired from international rugby with England in December and has recently been touted for a coaching role with the Lions.
But Gatland insists Wilkinson, who has been on two Lions tours, may travel as a player if he is fit and in form.
The New Zealander, confirmed as Lions boss last week, admitted appointing Wilkinson as a kicking coach may be an option, but refused to rule out taking him to Australia as a player.
“For me, everyone is in contention,” Gatland added. “The slate is clean for every player who wants to be considered.
“No one is ruled out or pencilled in. If you are playing well enough, then whoever it is – whether it is Jonny Wilkinson or whoever, it’s all about how players are performing for their clubs or countries.
“We’ve selected players who have been playing well just for their club sides and have done well for the Lions. They don’t have to be playing for their countries.
“What is important if they are not playing international rugby is that they have to be coming from a side that is performing particularly well.”
The Lions will play three Tests against the Wallabies and seven tour matches, including a warm-up against the Barbarians in Hong Kong.
Wilkinson has featured in five Tests for the Lions after earning caps in 2001 and 2005.