I just wonder why we have this discussion at all. Whether or not it is LEGAL to use the data produced by the NONCOMMERCIAL version of terragen for COMMERCIAL purposes (which includes the GPL), is one question. But even if it is legal - it for sure is wrong. Use the commercial version then, or ask the terragen people that question (maybe they will say that GPL is still okay to them, or offer you a registration that allows GPL redistribution at a lower price than the usual).
If people abuse their rights to no end, the consequence is that they lose their rights. Just look to the music industry. DMCA wouldn't have happened if people had kept their copying and file sharing at a moderate level. Games wouldn't usually have a copy protection which makes them not work on some systems if people would sometimes BUY games. And they wouldn't have anticheat software that takes full control over your system and lets attackers in if there weren't these pesky aimbotters who use an aimbot to get an advantage in a public or even competition game (IMHO there is nothing wrong with using one in a "fair" game, like, a LAN game where everyone uses aimbot, or where people KNOW about it).