The date of the Gūḍhapadā (updated)
Until today I thought that the work had gained little currency (only one ms. survives, no Tibetan translation) and was not at all influential (nobody seems to mention it or quote from it). I am happy to report that I was very wrong.
Leafing through the so-called Vanaratna codex (see Isaacson 2008) I noticed that the colophon (40r) of the Amṛtakaṇikā (henceforth AK), Raviśrī's commentary on the MNS, contains two verses not attested elsewhere (that is to say the mss. used in the Sarnath edition and the Cambridge ms., Add. 1108/13). I am not very familiar with the script, so I will not give the full reading, only pāda b of the first verse, which says:
'ślāghyā* gūḍhapadāśritādbhutabṛhatkāśmīrapañjī sakhā(?)'
[*make sure you read the comments by HI on how to construe this]
In other words Raviśrī not only mentions the title, but also tells us that it is a Kashmirian work. Moreover, he seems to have been a fan ('ślāghyā'* [see above]), and openly admits to have drawn upon it. Oh, and he also says that the work is 'massive'. Everything seems to match.
As far as I know Raviśrī's dates are not settled with satisfying certainty. However, he must precede roughly 1200 CE, because the Uddyota, Vibhūticandra's sub-commentary to the AK, by definition must have been written after the AK. The mahāpaṇḍita came to Tibet for the first time in 1204 (see Stearns 1996), therefore Raviśrī should roughly date to the middle/second half of the twelfth century or before.
Since our Advayavajra not only knows the Kālacakra, but also quotes lenghtily from the notorious Ādibuddha, he must date after roughly the mid-11th century. Therefore the date of the Gūḍhapadā must fall between cca. 1040 to cca. 1160 CE.
Well, maybe I should have entitled this entry 'The (very rough) date of the GP'.
Isaacson 2008 -- Harunaga Isaacson, "Himalayan Encounter: The Teaching Lineage of the Marmopadeśa (Studies in the Vanaratha codex 1)". (pdf) Manuscript Cultures Newsletter 1.
Stearns 1996 -- Cyrus Stearns, "The Life and Tibetan Legacy of the Indian Mahāpaṇḍita Vibhūticandra". JIABS 19.1.
UPDATE: One more thing. I have somewhat incautiously regarded the two verses transmitted in the Vanaratna codex as auctorial, simply because they sounded like it. Here is some further evidence to back that up: Vibhūticandra has some lemmata from the verse we are interested in, including the line mentioning the GP. The end of the Uddyota is unfortunately not very legible on the only ms.* I have at hand, which is Tokyo Univ. Lib. 18 (see for yourself here - you will have to navigate to the end of the codex by yourself). The Sarnath edition gives: ...... dapadam āśritā|
But if you squint a little you can almost make out: + ślāghyā gūḍhapadām āśritā| I would be happier if it read gūḍhapadā āśritā or gūḍhapadāśritā, but there we are. I think this shuts the case. The remaining question now is: why on earth did other mss. of the AK decide to get rid of these two verses?
*A plea: if you happen to have the other two mss. of the Uddyota (1. Āśā sāphu kuṭi DH 366, or 5254 in the catalogue • 2. NAK 3-655 = NGMPP A 117/10) I'd be very grateful if you could tell me what they read just before 'ślāghyā'.