The fact is that there was/is a long standing disagreement between the Greek and Russian Orthodox Churches over the acceptability of Baptism by pouring. In this matter, I think the Russians in the end are correct, and when push comes to shove they are justified since the Greeks never perceived this as so serious an issue as to break communion with a Church which was increasinbly being peopled (and ministered to!) by men who were "not really baptized."
As Juvenaly indicated, Baptism by pouring has long been considered a form of Baptism in cases of emergency. The fact that it can be accepted at all, warrants that the Orthodox Church can accept it in the case of the heterodox/schismatics by the same extreme economy. None of this changes of course, the proper, canonical method of Baptism.
As for the canons cited in support of the Greek/"Gregorian" view, I noticed that none of them were addressing the topic of "baptism by pouring", but the practice of baptism by single immersion, or the alteration of the Trinitarian formula as was done by certain sects (like the Montanists.) They were comparing/contrasting the proper, canonical form, with a specific form they were condemning. Hence, they do not stand as an air-tight argument against the Church accepting people (whether they were so baptized by those without the Church, or perhaps on their deathbeds but somehow recovered by God's help) via economy, imho (and obviously that of the Russian Orthodox Church for centuries on end.)
Even ROCOR herself received western converts in this fashion until 1971 (following the guidelines in the pre-revolutionary "Book of Needs"), and that change was not because of any "conversion from" the older practice, but was a reaction to ecumenism and the false inferences that were widely being drawn in the Orthodox world from the practice of receiving converts leniently. As OOD rightly points out, if this was such a big deal for Archbishop Gregory, it's incomprehensible that he would have sided with a Russian body to begin with - I'd also submit it makes incomprehensible his relationship with ROCOR to begin with (since if memory serves, he went to ROCOR from the Antiochians after 1965 - that would be back when ROCOR was still following the Book of Needs in receiving western converts from heterodoxy.)
Seraphim