Yes most brands of marshmallows that are sold in the USA are gluten free.
So if you're gluten intolerant you can usually eat marshmallows without any issue.
The pack of marshmallows should list if they are gluten free or not.
Not all brands of marshmallows are gluten free though but the majority of the marshmallows are gluten free especially in the USA.
I have a gluten intolerance myself and eat marshmallows at least a few times per week and they never have affected me.
So you could try eating the marshmallows and if they affect your stomach then they may contain gluten but most of them do not contain gluten so you should be safe.