You're asking some fairly basic questions again....
An isolation gasket is an electrically isolating gasket, with suitable washers and bolt casings which electrically isolated one part of a system from the other, sometimes used on pipelines where Cathodic Prtection is applied, to prevent the current leaking away to earth trhough flanged connections.
A guage plate is a plate of a specificed size - usually around 95% of the ID, which is run on a pig to demonstrate that the pipeline is at least that ID all the way round on the wentire length of the section. Usually is segmented to allow for passage even when dents do ocucr, but is a Yes/no inpsection tool, i.e. if there is significant damage to the plate, it doesn't tell you where it is, merely whether you have a problem along your pipeline / pipe or not.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
