Roberta,
The .gsm file type is the basic library part in ArchiCad. I would suggest you put it in a folder that you will be able to locate and that you could use for other library parts in the future.
Once you have done that, start ArchiCad and load your new Library folder in the File/Library Manager Dialogue box. Once the folder is loaded you will have access to the part as a object which you load with the "chair" icon in the Toolbox.
You should put in some profile information in your signature as the answers may be different depending on what version of ArchiCad you are using and what your OS is. See mine below.
Cheers,
Gerald
"The simplification of anything is always sensational" GKC
AC 25-4013 USA, CadImage Tools
2019 MacBook Pro 16" w/ AMD Radeon Pro 5600M GPU
OS X 11.6
2.4 Ghz 8 core i9, 64 GB RAM
27" LG 5K Monitor