Shalom, Sorry, but I have never came across a source that points out the level of where the righteous Gentiles going to be placed in Gan Eden. But I do know for a fact that the righteous Gentiles will rise at the time of the resurrection of the dead (Techiyat Hametim תחיית המתים) which is a huge reward one will receive as we will get to see and enjoy a very high level of G-dly revelation. We know that Gan Eden is a temporary place for the souls to be till the time of the resurrection of the dead, therefor, as long as a the righteous Gentile gets up at the time of the resurrection of the dead, that's what counts. After the resurrection of the dead all the nations will convert and be part of the Jewish nation all the best, Rabbi Alon Anava