Your expression seems just fine.
Can you please check your Qlik Sense release by going to your hub > About button (top right corner)?
Dynamic labels are available after Qlik Sense September 2017 Release. Check the release notes in this link.
If your Qlik Sense version is older than September 2017, then dynamic labels are not available yet for you. So your gauge chart is interpreting your expression as plain text.
You can also check it in your gauge properties. Does the 'label' input box have or have not a 'fx' button on the right?
If not, you may consider upgrading your Qlik Sense version to the latest version in your server.
Hope this helps :-)