If you see a warning "Cable gauge changes detected" on a DSL service qualification, this means that the qualification detected that the thickness of the copper cabling on the line changes between the exchange and the MDF/Site.
Changes in the cable gauge can affect line attenuation and reduce connection speed. In some circumstances this can also affect the reliability of the service if the attenuation is already of a lower quality.
This qualification result is a warning only, you may still proceed with the connection if you accept the risks involved.
Please note that this is not a provider-specific error - this test relates to the copper cabling in the street running from the local exchange to the premises. This warning affects all connections of this type from all ISP's.