It looks like it failed while trying to allocate a block of about 3 Gb (calculated from the 18k x 14k image size), and it looks like your system was already pretty close to maxing out your VM. But I don't have much experience debugging Mac crash logs so I might have this completely wrong.
This happened while doing the final GISD pass. It needs to allocate some temporary buffers to complete this step. Although the buffer it was trying to allocate at the time of the crash was about 3 Gb, it probably needed to do other similar-sized memory allocations around the same time, so even if your RAM use was well below the limit before the GISD step started it could still hit the limit here. As others have said, 18k x 14k is a very large image for Terragen to render, and few of us have experience rendering images of that size. It may have worked if you had more RAM in the system, or a larger VM allocation, but I'm not sure. It may have completed if you turned off GISD, but other post processes such as bloom and starburst may have run into similar problems.
If the scene had very large RAM requirements for objects and populations, then using smaller pops and/or simpler objects could have left more room for the final image processing steps to complete.
If my reading of the crash log is correct, you were leaning heavily on VM which must have had a terrible effect on render speed. If possible, add some more physical RAM to you system and you will probably render in a fraction of the time.
Matt