If you use a Mac, you may need to grant Gather permissions from System Preferences. Open System Preferences and select Security and Privacy.
Check both Camera and Microphone to see if your browser has been granted permission. If you have installed the Gather desktop app, you can also grant it permission.
If you want to screen share in Gather, in Security & Privacy, scroll to Screen Recording. Select the browser to provide permissions, or select the plus (+) sign to add your browser.