For a test method to be included in your scope as an ISO 17025 testing laboratory there are a number of technical requirements required. This includes ensuring the validity of results (clause 7.7) by monitoring an already validated method with the use of suitable checks. One of these is the use of reference material for quality control checks for your instrument and method. The requirement is that you can ensure metrological traceability, meaning all measuring equipment and the reference material must be certified.
Regarding the range – the QC checks must cover the calibration range / range of testing for each batch. As far as how often you should plan a QC check, it depends on the risk, including the stability of the instrument. I suggest you engage with your supplier of your GCMS for assistance on best recommended practice.