Alright, here's some feedback.

Thanks for adding linear gradients -- an essential part of UI design. The functionality of actually using them is great, but there are a few things you could do to help speed up the process of activating the tool in the first place.

Right now, it takes a minimum of three clicks to start using the gradient tool. That might not sound like a lot, but over the course of an hour, day, month, it adds up to a significant portion of time. Here are some base-level suggestions to improve the UX of this tool:

1) Assign a keyboard shortcut. I suggest 'G' since it's logical, used for other Adobe software and isn't being used by anything else to my knowledge.

2) Add it to the property inspector, or add it as a tool to be selected (I much prefer the first option). Either way, burying it within the 'Fill' menu seems odd given how important of a tool it is. It's definitely going to be used more than Shadow, yet that gets a headline spot and gradient doesn't? No way, man!

3) If the plan is to keep gradients within the contextual Fill section, then I'd suggest that you have it as a tab option rather than the dropdown it is now. That saves a whole click and avoids miss-clicking once activating the dropdown (micro-frustration).

More feedback as I use the tool more will potentially follow.

Credentials: UI/UX Designer currently using XD in real-world commercial projects.