Hi there, thanks for the question!
Yes, the assets are royalty free once purchased! You can use them in any number of projects, commercial or free.
The license for all of my assets is an attribution licence and can be read here: http://untiedgames.com/files/license.txt
It also specifies that you can't resell the assets themselves, only as part of a game or application product. (The GUI assets in particular are more general-purpose than my usual assets, so I'd allow their use in any type of application.)