With the Sassie glue gun you use Isomalt sticks. Isomalt does not stick well to frosting/buttercream.
Isomalt , being sugar absorbs moisture so it absorbs the moisture from the frosting.
If you were wanting to stick fondant decos on a fondant cake, it works great, but you don’t really need the gun to do this, just melt your Isomalt in a micro safe small bowl ( silicon cupcake wrappers are great to use) and dip your decorations into it and stick on. …Don’t forget to wear gloves it is incredibly hot!