GSM files are AC library objects. 3DS files are 3D Studio files. You can convert the 3DS files to GSM using the "Import 3D Studio file" Goodie, but GSM is ultimately the format you want. Associated image files could be for material textures, etc., so you may want to keep those with your GSM.
The biggest drawbacks to using these objects are that they usually contain an enormous amount of polygons, and aren't parametric (that is, you can't alter their size without distorting the shape of the object) -- but, of course, there are exceptions.
MacBook Pro Apple M2 Max, 96 GB of RAM
AC27 US (5003) on Mac OS Ventura 13.6.2
Started on AC4.0 in 91/92/93; full-time user since AC8.1 in 2004