I do not use GOG myself, so I do not know if there is a similar launch option control within.
However, you should be able to create a windows application shortcut to the Ashes_DX11.exe executable (right click and drag the Ashes_DX11.exe executable to the desktop, release right click and select "Create shortcuts here".
Right click the shortcut and under "Target", add "-modding" (without the quotation marks) at the end of the line after the quotation marks.
Example target line (application location will differ):
"D:\Program Files (x86)\Steam\steamapps\common\Ashes of the Singularity\Ashes_DX11.exe" -modding