Typically, it's easier to start from scratch then try to heavily modify an STL file. If the designer will send you the regular CAD files, those are pretty easy to work with in your CAD software of choice. For STL adjustments, Fusion 360 is really the only good option. You can convert an STL file to a solid body by right clicking on the part in the main design menu, then clicking "Mesh to B-Rep" or "Mesh to Solid Body". If the model is too complex, you'll have to re-mesh it first using the mesh menu to reduce the triangle count. That's a little more complicated, but there's plenty of guides online for Fusion that could walk you through it. Typically files with more then 30,000 triangles need to be reduced to under 30,000 to convert properly to a solid body.
All that begin said, gauntlets are generally pretty simple and you could learn to model one well having no previous experience in Fusion 360 in a few days. It's a free program with great user interface and a wealth of online tutorials for every aspect of the program.
