I installed the latest 4.3.0 and GCSM syncs the same contacts from Google to Outlook over and over. It never sees those same contacts on Outlook as being current. In addition, it is not syncing new calendar items FROM Outlook to Google. In other words, the program is simply not working correnctly at all right now. I also tried updating .NET to the latest version of 8 and that did not have any effect.
GCSM Version 4.3.0
Microsoft® Outlook® 2021 MSO (Version 2601 Build 16.0.19628.20166) 64-bit
Windows 11 Pro 25H2
And I just tried 4.1.34 and it does the same thing.
And tried 4.2.1. It doesn't try to sync all the contacts every time but it won't sync Outlook Calendar with Google.
Roll back to 4.1.33. Its the last version that works correctly. I have seen many bug reports of people saying the same thing and as far as I know, it has not been acknowledged by the devs. Hopefully at some point a fix will be released so we can all update past that version.
I've tried going with that version too and it doesn't work either. It has a tendency to duplicate contacts as well. I have switched to MyPhoneExplorer for now because it works. It's not as nice as a fully background process since it requires its own Android client but I have not seen any issues with the calendar, contacts or tasks. I donated to the dev here a long time ago and I'd donate again if the project had any signs of life. Sadly, it seems abandoned.