Wouldn't it be the 185 in Johannesburg - the "Great Escape" / "Great Survival"? Ray Illingworth called it "One of the greatest innings of all time". The basic stats - 185* off 492 balls in 643 minutes. Rest assured, Gambhir's latest 137 off 436 balls in 643 minutes (what a coincidence!) isn't going to be called as one of India's greatest but it does deserve a lot of applause. Gambhir, Sehwag's understudy (unbridled aggression) looked like Dravid's long lost brother or even Gavaskar's nephew!
When i checked Cricinfo Statsguru, Gambhir is in rare company. The list of players who batted 400+ deliveries to save a test or win it is an elite company of 24 innings by 23 players (Andrew Jones from New Zealand achieved this twice!). Gambhir's 643 minutes is the fourth (tied with Atherton) longest & apart from the Gordon Greenidge (226), the other three were match saving innings (following a follow-on declaration). Two have been featured in this blog. Care to know the others - the best was 275 in 878 minutes & 642 balls made by then-South Africa opener & current India coach - Gary Kirsten. The third longest belongs to New Zealand's own Mark Greatbatch who scored 146 in 485 balls and 655 minutes.
Gambhir, don't worry if this innings isn't placed in the same pedestral as Laxman's 281 or Gavaskar's 221 that nearly won the match (the other entries from India in the list of 24!), but your coach & team would know that your innings came very close. In fact, this one deserves to be in Top 10 Indian innings of all time . Also, one more small point - your Delhi teammate & stand-in captain, Virender Sehwag, owes you one. You (with copious help from the seniors) avoided the ugly questions for sehwag post his twin inning collapses against ordinary spin bowling. Encash it now!
PS: I played around the Cricinfo stats tool. To me, "Saving an away test match" is someone batting for over 500 mins (4 session) and 400 balls on foreign soil. That list is even more elite - just 7 "centurion savers"