The transformer is likely to be one made specially for Gossen. If you can afford a replacement from Gossen, that is the best solution. It may have higher insulation ratings then conventional transformers, or extra shielding.
It may be possible to substitute it with an off-the-shelf transformer functionally. If there is room in the case for a second transformer, it may be possible to keep using the original, and feed voltage into one of the secondaries from a second transformer. Any clues to voltages, like voltages marked on the PCB may help. You would need to work out how the windings on the secondary are configured, and then by applying 50.60hz AC to one of the secondary windings, measure all the ratios of the other windings to the first, along with the DC resistance of each winding. That would be a start anyway.
It would help if you can get a schematic, but I haven't seen many Gossen service manuals floating around.
If you want the meter though to perform to the original safety specs, you probably better go talk to Gossen.
Richard.