Very unlikely. Garnets form in metamorphic rocks, under high P,T conditions typical of the deep crust and mantle. To bring these to the surface is difficult, given the tectonic history of Mars. Perhaps the Hellas impact could have done this, but the products would then have been vulnerab;e to surface erosion and bombardment by smaller impacts..

Garnets also form in granites as primary igneous minerals, but granite is a highly-evolved silica-rich rock that requires recycling of crust and the existence of water to be formed. Again, Mars' history doesn't sit well with this.

Also, garnets are distinctive angular shapes, not rounded. Some of them, at least, should be in a less-worn state.