Gnome - windows always open top left

Actually, since GNOME v3.30 there is a visible option in GNOME Tweaks, which makes it much easier to enable it:

Just select "Center New Windows" under "Windows".

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Actually I found a solution for GNOME without compiz. You can either use

• this extension if you have gnome-shell < v3.14
• or open dconf and set center-new-windows in org.gnome.mutter to true: