Shortcut to change the gnome-terminal font size
Ctrl++ and Ctrl+- are working fine for me. Maybe you have different shortcuts configured.
To change the keyboard shortcuts, go to Edit -> Keyboard Shortcuts... in the gnome-terminal menu, and look for Zoom In and Zoom Out:
Try Shift Ctrl +for increasing
and Ctrl - for decreasing font size.
Note: the Shift key is needed on some systems.
Trick that worked for me was to use Ctrl + Shift + = or Ctrl + - but NOT on numeric keyboard.