I downloaded it using offline method, first checked file integrity with no errors, finished installation and get the following error: the code execution could not proceed because VCOMP120.DLL was not found.
Downloading if using Galaxy doesn't work either, as it is endlessly stuck on 92% complete download.
Does GOG offer refunds?
Concerning the "Vcomp120.dll missing" error, you can find the article linked below, to be quite useful. https://bit.ly/3j6b52T
In short, it is an error caused by not having installed the required "Microsoft Visual C++ Redistributable".
Its setup package usually can be found in a subfolder, located inside the game's main installation folder.
Otherwise you can find and download the, appropriate for your system, latest redistributable package, using the link below, https://bit.ly/35JBNq0
or by running a Windows Update scan, to (hopefully) automatically update your system's preinstalled "Microsoft Visual C++ Redistributable".
Related to the issue, where the GOG Galaxy installation gets stuck at 92% download progress, is the forum post linked below. https://bit.ly/3j2p9u9
Last but not least, concerning GOG.com's refund policy, GOG.com has a "no questions asked" one, as long as a refund request is submitted within 30 days of the purchase.
For more details on the matter, I suggest taking a look at GOG.com's refund policy aticle, linked below. https://bit.ly/3gZviEB