The names of seven days of the week exist in two forms, both attested from shortly before the beginning of the Christian era. One is the (originally) Roman planetary week, where each day is associated with one of the seven planets, which in turn have their names from seven Roman gods; this form is first alluded to by the Roman poet Tibullus in the first century BC. The other is the Jewish numbered week, with six days numbered from one to six, and the Sabbath on the seventh day; this is attested in Jewish writings from about the 2nd century BC onwards. It is thus not correct to say that the planetary week is older than the numbered week. In later Christian uses the two forms of the week are often combined. For example: Italian has giovedì “Jupiter’s day” (Thursday), but sabato “Sabbath” (Saturday).
It is generally accepted that at least some of the day names in West Germanic languages derive from Gothic. The main basis for this is that the word for “Saturday, Sabbath” is sambaʒtag in Old High German, Samstag in modern (Southern) German, etc., with dissimilation of -bb- to -mb-. This dissimilation occurs also in the word for "Sabbath" in Mediaeval Greek and in Slavic languages; it is thus assumed that Gothic borrowed the dissimilated form from Greek and then passed it on to West Germanic. There is a slight difficulty here, namely that the word for “Sabbath” in the Gothic Bible is in fact sabbato, not *sambato, but scholars have got around this by assuming that a popular form *sambato existed alongside the “learned” sabbato. A good synopsis of the standard theory is here:
As far as the Bavarian names are concerned, I think the page linked by you gives a very good account of these. erchtag, iarta, iada etc. come from Ares, the Greek planetary name for the third day (Latin Mars), and pfinzda etc. come from Greek pempte, “fifth”. It is just that I would have written not “durch gotische Vermittlung” but “wahrscheinlich durch gotische Vermittlung”, as the name for Thursday is not in fact attested in our meagre corpus of Gothic texts.
You can find a recent discussion of the origin of the seven-day week here: