At the moment the tool allows you to only kick off CI jobs when a branch is updated.
This is actually limiting the capability of the tool in itself,
because services like Bitbucket allows you customize trigger points, as to when they should ping the webhook;
such as the creation of pull request, merging a pull request.

The two trigger points mentioned above are specifically very important because you would want to run validation CI jobs on the creation of pull request or when the merge completes so that you can actually ascertain if your branch compiles.

Based on an update from Luke, it seems like the currently Gearset can receive the notification from source control for these triggers but it just ignores the ones that are not triggered by updating of a branch.
Seems like the best possible way (and probably the least time consuming too) to provide this ability to clients, for now, would be to

1. Let CI job run on the update of a branch like it is right now when you select - "when the source branch is updated" option
2. Add a third option - "trigger via source control" and trust the source control to manage the triggers points at their end and just kick off the CI job everytime source control pings Gearset.

Just a note that this should be available both for CI and Validation jobs both.

This feature is very critical for couple of our workflows and at the time we will need to have additional Sbox + branches to achieve this functionality or would have to validate this manually all the time.