The rabbis knew that the Earth was curved during the time of the gemara. They also knew about time zones and the effect that it had. The usages of the gates was a metaphorical reference. They could tell it was curved because they could (as an example) see how sailing ships dropped "below the horizon. When dealing with times that would have a different halacha in any particular area (such as when Shabbas starts or what time to daven) they use local solar time.
When dealing with a conceptual matter that can only occur once, they would use Yerushalaim time as that is the central location for Hashem's presence as I explain below. Since Gehinnom is a conceptual situation rather than a fixed location, the time in Yerushalaim is used, as explained below.
Note that in the discussion of makas bechoros the question is asked "Why did Moshe Rabbeinu say about midnight when Hashem said at midnight. While there are other answers, one of them is that Hashem has set the "reference time" to Yerushalaim. As an example we see
Parshas Bo 11:14
Hashem's 'clock' is set to Jerusalem. The plague of the first-born
took place at exactly midnight, Jerusalem time. But because Egypt is
west of Jerusalem, midnight there occurs later. When Moshe said the
plague would be at 'approximately midnight,' he was referring to
local, Egyptian time.
(Kehillas Yitzchak; thanks to Rabbi Sholem Fishbane)
Even though this explanation may have been given later, it is an explanation of why Moshe Rabbeinu said "about". That is, it is stating that Moshe Rabbeinu knew about time zones and the effect that they had.
Similarly, when we announce the Molad for Birchas Hachodesh, we announce it according to Yerushalaim Solar Time and not the clock time of that time zone (which is 21 minutes later).
Of course, the statement of relief from Gehinnom is a conceptual one, but this is the basis for that.