If you never build any units, never give the AI any GPT, and never build any city improvements, you will
never get a negative rate of income.
So you must be going over the top in one, or all of these aspects.
If you are a democracy (or republic pre-C3C), you get no unit support. This means for every unit you have you will be paying 1 GPT. Also, virtually every city improvement you build costs at least 1 GPT to maintain.
To avoid these problems, think about maybe changing goverments. Different goverments are good for different styles of playing. Also, don't build unnecessary things. When I first got the game, I would build stuff just for the sake of it - which was a waste of time and money. Set yourself goals, and aim for them

Don't build a huge military under a democracy if you are going for a Diplomatic win. Just like you shouldn't build lots of cultural improvemnts if you are going for a conquest win.
If you aren't going overboard in either of these aspects, check your trade agreements. Press F1, and have a look and see if you are paying any GPT to your competitors (should tell you up the top left of the screen)
If you are, stop it

If not, then you needn't worry.
Just incase you didn't know, you can adjust your tax slider. You will find this on the F1 screen in the top right corner. Put everything down to 0%, and I am pretty sure you will make a profit.
Hope that helps
