I definitely want this to be a feature.

But I worry that GearSet will have difficulty implementing this since their deployment package seems to be based on the diff between source and target. If you change the target during the compare process GearSet will actually throw an error that the target has changed and prompt you to rerun the comparison (while saving your selected items).

For now I use the "clone deployment package" feature, but it still requires you to wait while the comparison runs against the new target. Any items you selected in the original comparison while remain selected, but only if they are still registered as a difference.

This would help a lot because we often have to deploy to multiple instances

This would be really interesting to have. We have many sandboxes related to different tracks of the business that need to be backfilled with changes from production. Instead of having to clone the package several times, it would be helpful to have an ability to push from a source to multiple targets at once.