I have completely deleted all files and re-downloaded the game several times. I played very little of EA and never used Mods, I followed the instructions on Larians site but have not found anything. I need help on this one because when it STILL has mismatched files, and I am at a total loss as to how that could possibly be the case. Please someone help!

@CafeSpy what are the mismatched files? You can get the file names from open file list on the Data mismatch dialog button OPEN FILE LIST.
I had four files one of which was a GoG EULA. After 3 clean installs I deleted (not permanently to the recycle bin) the files. Then in the galaxy app clicked the icon next to play selected Manage installation/Configure... and the Verify/Repair and then rebooted my PC. Did not solve the mismatch still had one file being flagged Data\Patch0_Hotfix1.pak. But as that is the latest patch I just left it and play with the mismatch and hope GoG fixit on Monday.

The fault lies with GoG after all one file was their own EULA. I assume they have not updated the launcher that is why it see the patch as a file that should not be there.

True but I meant Gog are still installing the old launcher (or check list read by the launcher) not the new one.

I think I know what the problem is the file manifest is not being updated. C:\Users\XXXXXAppData\Local\Larian Studios\Launcher\Cache\4.1.1.3624901_galaxy_manifest.xml has not been updated since my original install before the hotfix. It does not have Patch0_Hotfix1.pak in the manifest so the launcher throws a file mismatch. 4.1.1.3624901_galaxy_manifest.xml is GoG's file that the launcher reads.

Yup. That did it ...Just know, for anyone looking to bypass the launcher in this way the exe file is not called an exe file. It is labeled merely bg3 and it is located in the Bin folder. But it does run the updated version. Nope I was mistaken. It does not run the updated version it is somehow still running EA version 4.1.1.364901 the updated release version would be 4.1.1.3622274

I fixed my launcher. I went to C:\Users\XXXXXAppData\Local\Larian Studios\ and deleted the Launcher directory and all its contents.

Then C:\Users\XXXXXAppData\Local\Larian Studios\Baldur's Gate 3 and deleted all the directories except PlayerProfiles (that's your saves).

Then in GoG Galaxy uninstalled BG3 then installed it again. The install finishes but shows as corrupted. I then rebooted and the Galaxy does a quick tidy up when I open it.

All works from launcher no mismatched files.

C:\Users\XXXXXAppData\Local\Larian Studios\ does not exist for me .I don't know why.

It is hidden. You have to make it visible in the windows options (IWin Explorer ->Options > View > Show > scroll down > Hidden Items) I´m German, so I dont know if I translated the path correctly :)

@ChrisDB44 Close enough the last bit is 'Show hidden files, folders' and drives.

It seems to be Larian's launcher that restores the files in the Appdata launcher directory, not Galaxy. So I think simply closing the launcher, deleting the Launcher's Cache folder, then restarting BG3 will suffice to fix it.

I ended up deleting all of it as per your suggestion, but instead of uninstalling the game, I just ran a Verify / Repair (and tbh I don't think that was even necessary) and afterwards my data mismatch error was resolved as well.

I think what may have gone wrong in this case is that on GOG the update was deployed in two seperate stages with the same version number (oops). And if you ran the game in between those two stages (like I did), the Larian launcher bumped your manifest.xml file upto v4.1.1.3624901 without adding patch 1 to the file list.

So we ended up with a manifest file that simultaneously had an out-of-date file list, but an up-to-date version number, resulting in the launcher not knowing patch 1 is part of the current game build. It's likely that this issue would resolve itself naturally with the next game update, since at that point the launcher should rebuild the manifest file anyway. But if anyone doesn't want to wait that long, then deleting the cached files and restarting the launcher should produce the same result, as without those files cached it will be forced to query the server or Galaxy to determine what the current file list should be.

This isn't a solution, its a workaround. GoG/Larian need to fix the false positives.

edit: checking the files listed in galaxy_4.1.1.3635601_alteredFiles.txt I had only webcache.zip and EULA.txt. I deleted them both from my game install directory, I no long have this issue.

Users really shouldn't have to do this...