The alt/option key is used in standard Cocoa/Carbon text fields to skip entire words when used with the arrow keys or delete entire words with the delete key. 
Neither behavior works in gEdit by default (it does with the control key, but that's not the default behavior in MacOS and old habits die hard)

Adding this lines to gedit.app/Contents/Resources/etc/gtk-2.0/gtkrc fixes it:

binding "gtk-mac-alt-arrows"
{
  bind "<alt>Right"           { "move-cursor" (words, 1, 0) }
  bind "<alt>KP_Right"        { "move-cursor" (words, 1, 0) }
  bind "<alt>Left"            { "move-cursor" (words, -1, 0) }
  bind "<alt>KP_Left"         { "move-cursor" (words, -1, 0) }
  bind "<shift><alt>Right"    { "move-cursor" (words, 1, 1) }
  bind "<shift><alt>KP_Right" { "move-cursor" (words, 1, 1) }
  bind "<shift><alt>Left"     { "move-cursor" (words, -1, 1) }
  bind "<shift><alt>KP_Left"  { "move-cursor" (words, -1, 1) }
}

class "GtkTextView" binding "gtk-mac-alt-arrows"
class "GtkLabel" binding "gtk-mac-alt-arrows"
class "GtkEntry" binding "gtk-mac-alt-arrows"


binding "gtk-mac-alt-delete"
{
  bind "<alt>Delete" { "delete-from-cursor" (word-ends, 1) }
  bind "<alt>KP_Delete" { "delete-from-cursor" (word-ends, 1) }
  bind "<alt>BackSpace" { "delete-from-cursor" (word-ends, -1) }
}

class "GtkTextView" binding "gtk-mac-alt-delete"
class "GtkEntry" binding "gtk-mac-alt-delete"