Gradient tool isn't working in channels.

Adobe changed the Gradient Tool, however, there are still issues that need to be worked out

such as the one you posted.

(under Improved Gradients)

https://helpx.adobe.com/photoshop/using/gradients.html

In the meantime you can switch to Classic Gradient in the Grandient Tool options and then

you should be able to draw your gradient in channels as with previous photoshop versions.

In Photoshop 23.5.5 this is what it looks like when doing the same thing....