It was a spiritual representation of Gamora within the Soul Stone
Recently, and I stress the importance of ascertaining the validity of these claims yourselves, co-Director of Avengers: Infinity War, Joe Russo, appeared at a high school and had a QnA with the students. These students apparently asked him about that place and Joe Russo confirmed that it was a spiritual representation of Gamora, within the Soul Stone:
[The next student’s audio did not come out clearly- from what I remember and from what I could understand- the student was asking why Gamorra appeared as a child towards the end of the film]
Russo: Yeah it’s- it’s implied it’s the soul stone. It’s all orange around, then he’s inside the soul stone with the amount of power that it took to snap his fingers- he has this out of body experience with Thanos. When he goes inside the Soul stone he has this kind of conversation with the younger version of his gamorra-
Student: So she’s there?
Russo: She’s in fact is yes.... It was an attempt an attempt on our part- because we don’t like two dimensional roles or three dimensional villains every villain is a hero in their own story and as insane and psychotic and brutal and violent as Thanos is he’s a more complex villain if you go on a journey with him emotionally. He does care for things and it is complicated for him to execute his plan and it cost him something. He said at the end “It cost him everything” and that it was the only thing he loved which was gamora which is why we put him back with her at the end. I just want to reiterate with the audience that he does feel true emotion even though he is a monster.
-Anon, [MASSIVE SPOILERS] Joe Russo confirms several question regarding IW at High School Q & A on Monday. Part 1 of Transcript in post., 2018-05-05.
[all mistakes are sic]
Gamora asked if Thanos succeeded, and what it cost him
As you asked how the conversation went; transcribing from memory, this is what happened there:
Young Gamora: Did you do it?
Gamora: What did it cost you?