Can I watch Gaia through the Apple App on Sky Glass TV?

1. If you mean the Gaia content provider, then they do have an app for the Apple TV box, but they do not stream through the Apple TV app as in-app channel on third party devices. They also have an app for select TVs and other devices.
2. If your TV of TV-connected streaming device supports AirPlay (some do, some don’t; see the list or the device manual), then you can stream from Mac laptop or iPad to the TV.
3. Through an HDMI adapter, you can physically connect your laptop or iPad to a TV. Not ideal imho, but possible.

no idea what Gaia is in terms of film or tv show but

the appletv app play appletv+ material which is a streaming service with apple produced material

read more here Apple TV - Apple

if Gaia is something streamed at another streaming plantform then you will not be able to do so

some appletv apps on some smarttvs also support purchasing and renting third party material and view stuff already purchased to ones apple account, so if Gaia is something you can buy or rent there then it really depend on what appletv app version your tv has.