Gated dedicated clock outputs

Apparently you are not on a dedicated clock output. To be sure, you'd have to check the pinout for the specific device you are using to find out what pins are dedicated clock outputs. Instead, you are using local routing to get to the output pin you have selected. The Warning you are receiving is just letting you know that you will have greater jitter on your clock output. To find out what that jitter is, you can use the "derive_clock_uncertainty" macro in TimeQuest. If you can live with this amount of jitter, then you can ignore the Warning. If not, then choose a different output pin.