When going to the Monitor > Machines page, the gateways tab does not appear next to machines and machine groups tabs.
Check that you have gateways by going to Data > Gateways and ensuring that the top right search field is empty.
A gateway will not appear on this page if it has been shared with you with the User permission. In that case you should ask the gateway owner or admin to check that the gateway has been shared with you.
The gateway pivot is only displayed if you own or have access to a gateway.
Create a gateway from the Data > Gateways page. The documentation for gateway creation can be found here.
Or ask someone to share a gateway with you by going to the gateway page, selecting the gateway and clicking on the Share button.
Then entering and selecting your name from the input field and clicking on the save button.
Then when navigating to the Monitor > Machines page the Gateways tab should be displayed. If it still does not appear it may be due to the cache then refresh the page and clear the cache.