A macron always indicates that the vowel is long. In the case of "gēns", the cases other than the nominative (gentis, etc.) have a short "e" in the stem, and so no macra, even though the two consonants ("-nt-") mean that the first syllable of each is long.
A long "ē" shouldn't be pronounced "ay" (especially by Australians). It should have the same quality as a short "e", but be longer. W. S. Allen's recommendation is to pronounce it like the French "gai" or German "Beet", if that's any help. The closest equivalent in English (and it's not very close) that comes to my mind is the vowel in "Cairns", said carefully so as to be distinguishable from "cans". A crude transcription: "rehks" and "gehns".
Also, note that the "s" in Latin is never pronounced like the English "z".