I don't think you'd use a gaussmeter but rather a mobile base station so you can measure the incoming pings This was the approach used by SRLabs when testing their Blue Merle software for IMEI switching

GitHub

GitHub - srlabs/blue-merle: The blue-merle package enhances anonymity and reduces forensic traceability of the GL-E750 Mudi 4G mobile wi-fi router

The blue-merle package enhances anonymity and reduces forensic traceability of the GL-E750 Mudi 4G mobile wi-fi router - srlabs/blue-merle