I am enjoying all the great comments about this topic. If I may I will add my 2 cents worth...
There are many types of gauges. Some are mechanical and not adjustable for gain, such as dial indicators, calipers, micrometers but may be adjustable for bias.
Others use transducers such as LVDT's, strain gauges, etc. and their signals are processed with computers or other methods (A/D converters and amplifiers) and give digital outputs. These are usually adjustable for both bias and gain.
For the first type, you can use artifacts such as certified master parts, gauge blocks, gauge rings, etc. to verify that they are measuring to the correct value and that they haven't changed. To me, that is verification. Depending on the robustness of the results, this verification may need to happen frequently (once per shift) or it could be once per day or week. The artifacts or masters used for verification need to be certified by a traceable lab on a scheduled frequency.
For electronic gauges, the electronics and mechanical settings need to be verified. Probes need to be mechanically set to the proper travel, as their linearity can be valid only over a relatively small range. This is best done with artifacts or master parts that cover 120% or more of the range of parts that will be measured. It is necessary for the gauge to be linear over more than the specification, because it is important to know the actual values of the parts that lie outside the specifications. I have used max, mean, and min masters for such gauges where the max and min exceed the spec limits by 20%. On the floor, this is typically called "mastering" the gauge, and unless probes are changed it usually only involves verifying that the zero(bias) and gain are correct. Many of these types of gauges will correct bias and gain errors automatically because the certified master values are entered into their databases. They can even recognize when the masters are loaded into the gauge in the incorrect order. If a change is made to correct gain and/or bias, I consider this calibration.
For CMM's, we verify them by measuring ball bars or ring gauges, and probes can be calibrated by measuring the artifacts and comparing the results to the certified values. The entire CMM is calibrated yearly by lazer alignment and compensating for wear of the mechanical elements. There are also standards available for doing this, such as ISO 10360-1 to 6. CMM probes are required to be calibrated because every time they are changed their position relative to the reference axis is slightly different.