I recently purchased Synergy. The server (v. 1.13.0) runs on a Mac (Big Sur). The client (also v.1.13.0) runs Pop_OS! 20.10 (based on Ubuntu 20.10) with the Gnome desktop environment. The client has no dedicated mouse/keyboard so Synergy needs to run at the login screen (i.e., before login). If I run Synergy after login (either starting manually or via the PopOS start-up manager), Synergy works as expected. The only problem is getting synergy to start before login.
I tried the autostart configuration recommended by Symless for GDM (with the necessary changes of folder `gdm3` instead