What's the best Germanic gateway language?
Would it be German or Dutch or something else?
Ignore English since we're already speaking it.
I don't think there is an easy answer. Dutch will give you easier access to German, but a lot less useful for Scandinavian languages. For those, Norwegian is probably the best gateway language. It's probably easier to list those who are not good gateway languages when coming from English: German, Icelandic (bother because of grammar), and Danish (pronunciation).
Being Belgian, at school I learned both Dutch (2nd language) and, later, German (4th language).
Dutch was far more easier than German (no declinaisons) and of very great help to learn German : neutral words (het) remain neutral (das), structure of the phrase is similar, verbs with particles work the same way, etc.. all without being ennoyed by those "der-den-dem-das" tables.
Most Dutch people -at least those living in a 50km wide strip along the border- learn easily and speak fluently German, while the opposite is not true. Not because Dutch is more difficult but because it is perceived as a less usefull language.