I think first of all you have to clear your situation. What do they want from you?
- It is very unlikely that they want you to learn an ancient language, because this now seems a dead end: there is a decreasing chance to find anyone knowing or wanting to learn G2, so the knowledge will be buried in the collapsing heap of code when the current scientists leave or the all-patched code fails more and more often.
- Are the scientists (or some of them) ready to learn a new language and lots of programming paradigms? Or do they want to separate the programming and scientific activity in the long term, and perhaps have some more programmers if needed? This seems a rational and more efficient separation of expertise.
I think the core requirement here is to "save the knowledge in the system", so you have to go and excavate it!
The first task is to write a documentation.
Analyze the structure and the requirements as if this would be a new task, but with the help of an existing system. They will be pleased because you ASK instead of TEACHING first - and you will quickly get enough, but more organized background knowledge from a programmer's point of view: "what's going on here?" The docs (system static structure, workflow, components, problems) will be immediately valuable for them, and will perhaps show up more relevant information to them than to you (some of the guys may have "AHA!" and start fixing some codes immediately)...
You should then start asking where they want to go?
If they are ready to move away from G2, what system do they want to see (platform, language, interface, general structure)? You might start writing an external wrapper around the system if at all possible, having the target structure, but keeping the original components, thus slowly starting a kind of framework that allows new components to be implemented in a this target environment. You have to find the core services (persistent data connections and "toolkits": core calculation, drawing, ... libraries), and so you provide them a familiar environment in a new platform and language, which allows the transition either by you or them: take the old codes one by one, reimplement (and CLEAN!) them in the new environment. When that is ready, they know the new language; and the service layer (mostly made by you, sorry) is ready to host the new components.
If they don't move, you must learn G2, and create the modular framework there, into which you or they should move the components (with cleaning). Anyway, the language is just a serialization of data and algorithm tree...
While analyzing and writing the docs, read, use and advertise GoF Design patterns! :-)
... my 2 cents