Go to https://extensions.gnome.org/extension/935/toggle-touchpad/ and install from there.
Make sure you have xinput installed (sudo apt-get install xinput).
Download latest release (https://github.com/mkopta/toggle-touchpad/releases/download/v1.4/toggle-touchpad-v1.4.zip) and unpack it to this directory ~/.local/share/gnome-shell/extensions/toggle-touchpad@martin.kopta.eu. Restart the gnome shell by ALT+F2, type r and enter.
Commands to install
rm -rf ~/.local/share/gnome-shell/extensions/toggle-touchpad@martin.kopta.eu
mkdir -p ~/.local/share/gnome-shell/extensions/toggle-touchpad@martin.kopta.eu
cd ~/.local/share/gnome-shell/extensions/toggle-touchpad@martin.kopta.eu
curl -LO https://github.com/mkopta/toggle-touchpad/releases/download/v1.4/toggle-touchpad-v1.4.zip
unzip toggle-touchpad-v1.4.zip
rm toggle-touchpad-v1.4.zip
Now ALT+F2, type r and enter.
Make sure you have xinput installed (sudo apt-get install xinput).
Use gnome tweak tool to see whether the extension is really there and enabled.
Use looking glass to see more details (ALT+F2 lg)
(icons taken from https://github.com/orangeshirt/gnome-shell-extension-touchpad-indicator)