Yep, I had the same thing happen to me. I found it odd how the one empire expanded at the rate that it did, and when I went to go speak to it, I noticed the negative Gildar bit. Unfortunately, you cant take advantage of the glitch yourself. I offered all my Gildar for the entire negative Gildar debt that he had. I got the negative debt, and he retained his debt... However, I couldn't actually do anything with the debt. Apparently us humans aren't worthy of such exploits. 
It's not really a huge problem for me as I've isolated myself from the empire using the flood spell. I left a small choke point that he's more than happy to funnel his troops through. My company of elite dragons then wipe them off the map. Meanwhile I research spells and got a grasp on the game mechanics.
I'm not sure if the problem can still manifest itself in the day 0 version, or if the day 0 patch will render said empire crippled when the save is loaded yet. If it can still happen in the day 0 patch, then yeah, it needs to get fixed ASAP. If not... no biggie, as I wasn't expecting the game to be critical bug free until the day 0 patch anyhow.
Edit: Day 0 patch doesn't fix the issue in existing saves that have the problem. He's still able to create armies and new cities/buildings despite the debt. Still no idea whether the bug can actually occur in the current version though.
At any rate, it seems the AI doesn't have all, if indeed any, of the sanity checks that the human players get. This is no doubt under the assumption that the AI can't and won't make decisions that would lead to normally impossible situations. By not having the game logic cross checking every action the AI takes, they can conserve CPU cycles. Introduce some bug that creates said impossible situation, however, and the AI runs rampant with nothing to stop it.
