This discussion has taken an interesting tangent...
I'm no expert by any means but I did want to raise one other alternate theory. I have seen unworked "gigi" in Kelantan and Pattani and it doesn't resemble at all the hippo ivory I've seen. Rather it was represented to me as walrus tusk (gigi singa laut). Based on this experience I've always believed that gigi = walrus ivory.
Comparatively, you see a lot of "gigi" in Pattani, Kelantan and Terengganu. There must have been a reliable and fairly abundant source, but the Malay world's ties with tropical Africa were pretty distant. That's not to say that some hippo ivory didn't make it there but it is very rare IMO, having as it must to travel through the traditional trading route along the African and Iranian coast, through India and then only making the jump to Sumatra, Kedah then overland (or later, through Singapore). I've only seen one hilt in Kelantan that I'm fairly confident is hippo...this is based on comparison with photos of Persian khanjar hilts I've seen.
However with the walrus hypothesis you may have a source for the material. Its been reported that in the 18th and 19th centuries, American whalers sometimes wintered by sailing a loop down into the S. China Sea, following the coast of Vietnam and the Malay Peninsula before turning back east along the N. Borneo coast. They brought with them mostly whale bone and hippo ivory to trade for supplies and probably some exotics to bring home (some to the Peabody Essex apparently
) . Popular tradition in Kelantan is that the gigi was bought from American sailors.
Anyway, perhaps someone with more expertise on the material side can do some digging and see if they can find comparisons to walrus ivory?
Also, as an aside...another sea mammal here in Southeast Asia worth looking at is the dugong or sea cow. I was told by the carvers though that dugong ivory is too small to make proper "hulu gigi". Also, dugong are very rare, shy creatures which doesn't jibe with the huge supply needed to make all those gigi hilts. I've seen some badik hilts that were said to be dugong though.