Apologies for the long post but I thought I should document my adventure with GP3, just in case it happens to anybody else. Once again, thanks to all who offered me valuable advice on this thread, I learnt a lot :-)
Just to wrap up, I celebrated too soon... subsequent attempts came out a disaster (see 1st pic "test-2"), especially in the darker areas. The dark tones came out "blotchy", light tones fared much better. Most shots had the numbers/dots imprinted into the film.
Anyway, I tried five rolls before I finally realized... it wasn't me, it was the film !!!! My friend passed me these rolls and he had them for about a year. He shot two rolls and one had the same problem as I did.
It turns out that GP3 really doesn't store well in hot, humid climates like Singapore. This problem was also mentioned in another site:
So, for the benefit of others who may face a similar problem, poor storage (in our case, film was kept for about a year, under tropical conditions, in a non-airconditioned room) may lead to the film looking a) blotchy in the dark areas; b) imprinting of numbering on the film and c ) film comes out looking "underdeveloped", as per my initial shot.
Anyway, I've shot a roll from a another batch and this time, 16 mins with agitation every 4 mins works fine (see 2nd pic "symmetry"). No numbers imprinted on the negatives, and no blotchy dark tones..
Rest of the roll can be viewed here
p.s. for those who are stuck with lots of GP3 with this problem and don't want to throw it away, you can try extending the development time (in my case, I went as far as 24 mins with 5 secs agitation every 4 mins) and you can improve the image quality somewhat, even though the numbers will still be imprinted on the film (see 3rd and 4th pic "scan 1" and "scan 2").