Some experience here:
I have some Garuda miles and thought I make use of this ripper deal. Unfortunately, the miles were not enough for redemption of 2 tickets to our destination Narita. I thought I might as well transfer some from Citi Prestige and rung the Indonesian call centre. After about 8 tries, got hold of CSR and found the following things:
1) Garuda Indonesia would not allow transfer of points from Citi Prestige if it is issued in Australia. She gave me a list of 6 countries (Ind, Indonesia, Sin, Malaysia and some others) and they only have a tie up with Citi in these countries. (I thought I might as well ring Citi Australia to confirm this, but only if I could find available seats).
2) This redemption could only be awarded from Jakarta call centre
3) Availability: Ample availability from Mel-Jakarta (1 J and 1 F) both legs, but no availability whatsoever between Jakarta-Narita (both legs) and DPS - Japan (Both legs). This was for set of dates in Apr/May 2017
4) She was quite happy to hold/redeem the booking for the first leg. (had 38546 miles in account) - So could have been a great F/J redemption, but really needed to get to Japan
It is some what a moot point now that they have pulled the plug, but some info regarding transfer from Citi. Haven't rung Citi to confirm whether that is actually true though.