The only mgc to mgs conversion that makes sense, is potential export of content done using the MGCSoft Equation Illustrator, which is actually the software that uses both of these file types.
Unfortunately, it might very well be that the software was discontinued or the company no longer sells it. But it is unlikely any other software can help you with this.
Of course, this might also be a typo or used for conversions not related to files, for example unit measurement.
Updated: October 27, 2023