According to a comment by @SebastianKoppehel, the interpretation of porta itineri as "the gate to the journey" seems questionable. Wiktionary, for example, has the following translation:
The gate to the journey [is the] furthest.
Is this the correct interpretation or should it be understood differently?
For an alternate understanding of it, see the entry for porta at Zeno.org:
der Weg bis an das Tor ist der längste.
The road to the gate is the longest.
The quote is an adaptation from Varro's Res rusticae 1.2:
Sane, inquit Agrius, et simul cogitans portam itineri dici longissimam esse ad subsellia sequentibus nobis procedit.
The English translation by W. D. Hooper and H. B. Ash is available online and reads:
"By all means," replied Agrius; and reflecting that the longest part of the journey is said to be the passing of the gate, he walked to a bench, with us in his train.
This translation is fluent English and communicates the idea well, but the question of proper grammatical parsing of the dative remains.
I would understand the dative itineri as "concerning the journey" or "to the journey". Perhaps a somewhat literal translation of porta itineri longissima est could be "the gate is the longest thing to the journey".
One option is to read it as a possessive dative. That is a simple and attractive choice but requires some filling in: "[getting through] the gate is the longest [part] of the journey". Both readings lead to the same meaning so it makes no difference although the exact grammatical role assignment is different. And any reading seems to require supplying some implicit words and this is a weird instance of the dative, so there does not seem to be such a thing as a literal translation.
some additional information (work in progress)
Robert H. Rodgers, who has been working on a new edition of Varro’s De Re Rustica, writes that “we have here a second proverbial expression” (Rodgers 2015, p. 170).
Rodgers claims that “in direct speech the thought would then be porta itineri longissima est, ‘in contemplating a journey, the gate is the most distant point’” (p. 170).
He also mentions (on p. 171) that
“Giusta finds the dative forced (he would have expected a genitive), and sets forth a more complex alternative: porta interdici longissimum esse ‘è lunghissimo essere trattenuti fuori della porta'; he explains that our guests are obliged to wait outside the temple as travellers might be delayed in formalities at a city gate.”
cf. Giusta "Una congettura abbastanza semplice è porta[m] interdici longissimum esse, «è lunghissimo essere trattenuti fuori della porta», ossia «non potere entrare in città»" (NB: note his conjectural emendation porta[m])
Rodgers writes that this suggestion proposed by Michelangelo Giusta “is not unattractive in its meaning, nor unreasonable from a paleographical point of view” (p. 171). He mentions two similar constructions: longum est meaning ‘it is tedious’ and longissimum spatium + gerund (Rhet. Her. 4.53, Var. LL 6.11).
Finally, Rodgers admits that “if the dative is troublesome, one could maintain the traditional interpretation, less intrusively writing <in> itinere, ‘in the case of a journey’” (p. 171).
Next, I am going to read Giusta, Michelangelo, Raffaella Falcetto, and Giuseppina Magnaldi. 2006. Per il testo delle Res rusticae di Varrone: libri I-II. Alessandria: Edizioni dell'Orso - and I'll add more info then.
