Remmina - The GTK+ Remote Desktop Client  v1.4.2
Remmina is a remote desktop client written in GTK+, aiming to be useful for system administrators and travellers, who need to work with lots of remote computers in front of either large monitors or tiny netbooks. Remmina supports multiple network protocols in an integrated and consistent user interface. Currently RDP, VNC, NX, XDMCP and SSH are supported.
How to configure key mapping with the VNC plugin.

Remmina has an hidden feature with which you can configure custom key mappings, this is useful if you need, for example, to map the local Super key to the remote macOS Option key *(leaving theexisting local Alt key to the remote macOS* Command key map alone.)

See #1716 for a long discussion about this.

Keys can be mapped according to a configurable map. There is a predefined map called "Map Meta Key" that seems to be exactly what we need for macOS.

Edit your connection profile, under "Basic settings", you will find "Keyboard mapping". Choose "Map Meta Keys".

image

Save and connect.

Map Meta Keys is the following map:

[Map Meta Keys]
Super_L = Meta_L
Super_R = Meta_R
Meta_L = Super_L
Meta_R = Super_R

Other maps, using the same syntax, can be created in the remmina.keymap file, located in the same directory as the remmina.pref file, usually ~/.config/remmina/remmina.keymap Key symbol names can be found on the GDK file gdk/gdkkeysyms.h.