This question: Story about aliens nicknamed 'Eechees' who have created a network of tunnels on Mars, raised an issue about duplicates. (The querent has not confirmed/accepted an answer yet, but there can hardly be doubt that they are asking about Pohl's Heechee Saga.)
The reason that I saw fit to write a whole new answer was that, while most of what the question describes is pretty clearly from Gateway, the last two points are not. At least in the English edition, they refer to content from the second book in the series, Beyond the Blue Event Horizon. As I state in my answer, the most likely explanation is that there was a French edition of La Grande Porte that included additional "teaser" material at the end, drawn from the final chapter of Beyond the Blue Event Horizon. (Alternatively, they may have forgotten about reading the second book, but that seems less likely, since they did not mention anything about the main plot of Beyond the Blue Event Horizon in the question.)
Should this still be considered a duplicate, if it can eventually be documented that it came from an edition of La Grande Porte that actually different in terms of the material covered from the English Gateway? (Adding to the complication, there are already different versions of the English Gateway. As described here: How do the versions of Frederik Pohl's Gateway differ?, some editions include a final denouement that was not there in the first novel printing.)