Markus:

Chassis punches would do it, but are expensive
Drilling a series of small holes around the circumference, and then cutting/
filing is cheap but tedious
A tool called a nibbler would work. This allows you to enlarge an initial drill
hole. Also a bit tedious unless you have a power nibbler.
Use a jigsaw with a metal cutting blade, but might bend the sheetmetal.
A light touch with a cutting torch?
Large caliber artillery is probably the best as it will roll the cut edge
making it cut proof, but these are expensive, and likely illegal? Besides,
it might be hard to get a caliber to match your gauges;-)  

>Hi,
>
>I'd like to cut some circular holes in sheetmetal for gauges.
>
>Does anyone know how to create these kind of holes?  I know how to do it for
>wood, but I'm not sure this would work for metal.
>
>Any help would be appreciated.
>
>..Markus