This is a bit of a dusty one I picked up and cleaned off and decided to use. I exported the default ones as Obj for quick use, but if you want you can open the blend file and grab the optimized ones, or even higher poly ones. I am also aware some of these grenades never existed before but this was originally a fun what-if project. Reimagining grenades for various time periods/genres/eras/so on I may do another one in the future as well.

Technical Info. All images/textures should be good to go as well most are AI-generated or handmade.In the blend file there is a set with a custom collision box built just name it and attach it for your game engine, if you so desire it.
All OBJ files should be zeroed out at world origin as well. The Blend file looks as it did rendered. However the origin point is assigned to all in an appropriate manner so it should be easy to zero any out that you need. The collision boxes are all in special groups but not parented though since I did not know which engine anyone would use for this so left it up to your descrition how to attach it. Optimized models are technically lower poly models but I would hesitate to call them fully low Poly I tried to mantain the original in as much detail as possible while reducing redudant polys so it may can go lower but if it does it is likely to lose some details. In this one Materials could be optimized a wee bit more using various methods admittedly but this is was an entirely different project originally I opitimized it down as much as I could.