I don't realy know the answer so I hope "geeves" or someone else more knowledgeable jumps in but I have been going through simmilar things with my 1988 4JB1T. I got a belt to do some maintainance and then found out it is gear drive, anyway yesterday while I was down in Perth I went around to a 4WD wrecker and they had a 4JB1 and what was surposed to be a 4JG2 sitting side by side on a pallet and they were "both" gear drives ! I don't know how to tell the two series apart, but that is what the bloke I was talking to reconed it was. If its got a steel cover over the timing chest aparently its gear drive and plastic means belt drive. The black coloured steel cover of the gear drive has a sound deadening material under the cover between it and the aluminium front plate that encloses the timing gear set. I found it has a "lump" in the cover that corresponds with where you would need a boss for the bearing that carries the idler gear between the crank and injection clearly visible in the front. If its got this lump and a metalic cover likely gear drive, plastic cover with no lump = belt drive.
I got some answers on this site when I asked about my problem several days ago and I think one of the posters said they had heard that there were gear drive 4JG2s but that he had never seen one so they must be probibally early versions as with the 4JB1s the gear drives are the first and then the belt drives came later.
I must check out my thread when I finnish writing this and see what was said, sometimes my memory is a bit iffy.