GSRT stands for "Grandstream Ring Tone" and is a proprietary audio format used by Grandstream IP phones. The GSRT format is a compressed format that allows for efficient storage of ring-tone files on the phone. To create or customize a ring-tone in the GSRT format, you can use the Grandstream GAPS (Grandstream Auto Provisioning System) or the Grandstream Ring Tone Generator tool, both of which are available on the Grandstream website. These tools allow you to create and customize ring-tones in the GSRT format and upload them to your Grandstream IP phone. If you have a GSRT ring-tone file and you want to play it on a computer or other device, you will need to convert it to a more widely recognized audio format, such as MP3 or WAV. There are many audio conversion tools available that can convert GSRT files to other formats.