It used to be possible to breed negative BLUPPED parents and have the GP go down instead of up. They changed that ages ago but there's still a few kicking around. Now the only ones that go down are donkeys; which is how they are designed.

When it was possible (before July 2012), we started with crossbreds from 350 foundies. I'm not sure why crossbreds were chosen; I have a very, very vague memory that one of the originators of the Zero/Negative GP Project was thinking that crossbreds would get her to low GP faster, but later experiments using purebreds showed that they lost GP at the same rates under the same conditions. By that time, though, the crossbreds were kind of ubiquitous!

Prior to July 2012, Foal Games gave GP points instead of skills points.

Also prior to July 2012, if you bred horses in negative BLUP, their foals lost GP.

So we started with 350 GP foundies, crossbred thinking it would help (again, purebreds had about the same rate of GP loss per generation), and then each generation we skipped Foal Games to avoid the extra GP points, and bred the horses at 2 years 6 months of age to get the best possible GP drops. Lather. Rinse. Repeat.

Changes to the game in July 2012 made all of that impossible to continue with. If you breed existing negative GP horses today, you'll get foals whose GP goes up, not down.

Yes, if you breed today from existing negative GP horses, and if their GP is low enough, you absolutely can get new negative GP foals. But those foals' GP will be higher than what you would have gotten before July 2012, and if you keep breeding the line forward the GP will just go up until you're back into positive GP again. The only way to start in positive GP now and get to negatives is to use a horse with very very low positive GP, and breed to existing negative GP horses, the farther into negatives the better. It does not work now the way it used to before those game changes to breeding and foal games were made.

Thank you for the excellent answers!