Gunzel is considered mildly derogatory though is also used as a term recognising a level of interest or knowledge. It can be both in the same sentence. As an example if one is able to quote the time of the next three departures from Little Nuttingford the reply might be "You gunzel!".
Those working in the rail industry might use the term to describe others (fellow employees or otherwise) and its use is the same; mildly derogatory but often with a hint of respect.
It is sometimes applied to those with advanced and specialised knowledge though usually in those instances it is out of respect. I have heard the term "Senior gunzel" used to describe one or two who could, indeed, be said to have achived that state by dint of publications and general knowledge.
Other occasional variants are "Professional gunzel" (one who earns money from the hobby by some means - possibly as a commercial photographer / author or perhaps as a known enthusiast train driver), "Bunzel" for a bus enthusiast and "Tram gunzel" but never "Trunzel" for one who follows that mode avidly. On a couple of occasions I have heard the term "aircraft gunzel" used to describe those with a keen interest in aviation or the observers just outside the fence at Tullamarine on the Sunbury road.
The UK term "gricer" (soft C) is broadly equivalent though is typically applied to an enthusiast who travels rather than a platform-ender. "Basher" is used to describe a gricer who only or specifically travels behind particular locomotives / classes or who is intent on travelling all tracks in a given area / country (also "Track basher") while "Anorak" is definitely a derogatory term aimed at usually platform-end spotters whose chosen style of dress was often an anorak back in the 1970s. Attire has moved on but not so the language. For road transport the term "Crank" is used to describe an enthusiast while "Cranking" is the act of travelling on a particular vehicle or route and equates to the Australian "Bunzel". Thus you might crank the Routemaster bus on London route 15 then grice a spark from Charing Cross to Croydon before cranking the trams there.
Another term in the UK is "Bellowers" which refers to those who hang bodily, and dangerously, out of open windows often with arms outstretched to their farthest limits and who are either considered to "bellow" (shout) about the wonders of the locomotive they are riding behind or are listening to it "bellow" as it works hard. Bellowers have a bad name among rail operators and other enthusiasts alike. They first appeared on the scene in the 1970s as certain diesel classes were being withdrawn and became a significant nuisance in the 1980s. They still exist but with the replacement of older rolling stock by air-conditioned fixed-window types, and better stewarding on charters, their numbers have dropped in recent times.
Australia uses the term "Caders" as an abbreviation of motorcaders for those who chase trains by car without riding them usually for photographic purposes. In the UK such activities are less common but you might encounter "Chasers" if there is a steam train running. One difference is that in the UK many rail lines do not have parallel roads making the chase harder.