Think of the GDP metric as representing growth of GDP (or decline) instead of pure GDP. The design of the game would rapidly collapse if none of the values were ever capped, because we represent happiness of voters on a percentage basis.
I should probably just rename GDP to 'The Economy' instead.

Originally posted by The Pope:

This game would be much better if GDP was not a static metric and it had a growth rate that could be affected by policies, adequate political simulation, in my opinion, requires this.

Experimentation with mods of soft uncapped GDP (ultra-high boundary), in my experience have led to little material difference to the game.