If you utilize the tags feature of NextGEN Gallery, then this video might come in handy.
So as you can see, you have the ability to insert a gallery based on the tags that you're using.
You can select one, a few, or all tags if you really want to. But the fine-tune control can be very effective depending on your goal.
Thanks for checking it out,
Update 5-18-2020: We had a question in the comment below about tag clouds. I decided to answer it in a video so everyone can see other methods of galleries based on clouds.