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I recently purchased the War for the Overworld: Heart of Gold downloadable content through the GOG Galaxy client. Upon completion of my purchase, I attempted to install the DLC. The client showed me the installation bars, percentages and the like, but it went quickly, and I barely had time to register what had happened before it was finished, stating the game had updated. I was still being prompted to install Heart of Gold however, on the right side of my library menu, where other DLC for the game was suggested. Nevertheless, I had confidence in the game, and ran it, only to find the DLC had not been installed, and if I wanted to play it, I would need to purchase it again, through the link provided in game. Confused, I closed the game, and ran the installation again. Still nothing. I ran the installation through the settings, only the checkbox would not stay marked. I verified and repaired the program through the settings. I finally gave up and resigned myself to uninstalling my game and reinstalling it, and when it was done, it stated the DLC was installed, the checkbox was marked, and next to the version number on the library page, it no longer said 0 DLCs, it said 1 DLC. I eagerly opened the game, only to be presented with the same issue. Frustrated, I uninstalled the DLC through the settings page, and when I tried to reinstall it, the checkbox again remained unmarked. I've already tried support with this, but I'm still waiting for a response, still, I thought it couldn't hurt to ask you guys if anyone else has or had this problem, and what steps you took to solve it.
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Thats a bug in galaxy.
just take a look in that thread
GOG will it be fixing in the next week and there is a workaround inside.