I just found out the hard way, but If your gold reaches 0 and you have negative gpt, the game will start disbanding 1 unit per turn. I saw no mention of this in the manual or the civilopedia, they only mention the loss of science. So I was quite surprised when a pop-up appeared telling me my cannon had to go bye-bye because I had no money. This happened late in the game where I was aiming for a cultural victory and my economy started to crash. I had already researched Globalization and was waiting for the last few policies to come in when my golden age ended, my income dropped to negative, and I had just spent most of my gold as gifts to city states. I figured I didn't need science anymore and made the decision to just let the economy tank and wait out the win. Then I started losing units. Don't make this mistake.