Keeper - two things are important here:
1. Technically only Soledad sees the glow from the detect magic spell. I didn't make that clear in the post, but that is how the spell works. So she would need to tell you about it and then you could try to get it. If she chooses to do so it would only be a delay of a couple of seconds.
2. The gnoll is in midstream. Swimming is something you can do and if it was just an easy swim in a lake I wouldn't worry about it. This is a river with a reasonably swift (Not crazy fast, but not lazy either) current. The gnoll is midstream and moving away from you. You can run down the shore and catch up to it, (Difficult terrain doesn't quite start immediately.) or reasonably close, and then swim out to it as it continues down stream. You are wearing leather armor which isn't a big deal but isn't nothing - and you are also carrying other goods as well. In other words, you are moderately encumbered. So if you choose to go after the glowing object I woild do the following:
A. Allow you to reach the gnoll with no major issues. But exactly how far downstream it is by the time you get there is harder to tell.
B. You would then have to either drag the gnoll to shore or try and acquire the glowing object - which would no longer be glowing FYI. Either of these would require an athletics check to keep swimming. Possibly more than one depending on which choice you make and what you are trying to get. (Soledad only saw a generalized glow.)
C. Getting back to shore would also require an athletics check - you would be fighting the current to some degree.
It is also worth noting that there is a bend in the river coming and it is reasonably possible that you won't get to the gnoll and back before going around the bend. That doesn't mean anything beyond being out of sight of the rest of the party at the moment, but it is worth mentioning.
Successfully doing this is reasonably likely, but not guaranteed. The exact consequences of a failed athletics check might not automatically be drowning, but obviously it wouldn't be great for you. Let me know what you wish to do.