Using Alt key like the mac command (⌘) key for copy/paste

I'm looking for the same answer, but I haven't found a good solution yet.

In the interim, I've swapped my alt, ctrl which helps with keeping my muscle memory.

This might work for you in the mean time:

setxkbmap -layout 'us,us' -option 'ctrl:swap_lalt_lctl'

And you might have to keep Gnome from resetting your layout:

sudo gsettings set org.gnome.settings-daemon.plugins.keyboard active false

Hopefully someone a little bit more knowledgeable will provide a solution to keep from using Shift+Ctrl+c in the terminal.

[EDIT] Turns out the settings don't last past a reboot. I set this in my xorg.conf

Section "InputDevice"                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
  Identifier     "Keyboard0"                                                                                                                                                                    
  Driver         "kbd"                                                                                                                                                                          
  Option         "XkbRules" "xorg"                                                                                                                                                              
  Option         "XkbModel" "pc105"                                                                                                                                                             
  Option         "XkbLayout" "us"                                                                                                                                                               
  Option         "XkbOptions" "ctrl:swap_lalt_lctl"                                                                                                                                             
EndSection