The goods they consume aren't counted, and also most of their income. Look at a peasant anytime (outside of corner cases like with Workers' Coops.) Most of their income is this "subsistence income" which isn't taxed and doesn't contribute to the displayed GDP in any way. It's basically impossible to accurately estimate the GDP contribution of peasants in the game as it currently exists.
That's hardly even the biggest issue. Sure, the contribution of peasants is underestimated, but many other things are overestimated. Making a direct comparison of the Green Line Product and any real-world GDP is nonsensical. You could try to estimate GDP via income, if you have both income tax and dividend tax in your laws, but that still misses some things. "Subsistence income," as previously discussed, and dependent wages too, unless I'm mistaken. There's possibly other inconsistencies as well, like wages paid from nowhere. I've been told that businesses can potentially continue to pay wages even after their money pit empties out. I think this is due to the fact they can only lower wages once a week, like 10% at a time, and don't start until the money pit is at least half empty. In perverse circumstances (i.e. the player making very drastic, probably inept, PM transitions) they won't be able to respond in time.
It's not immediately clear what effect the "leaky pockets" of pops has on this, either. Here, I'm referring to the fact that excess income, beyond what pops spend on needs, doesn't go anywhere. It just disappears, effectively. And depending on the relative value of the excess, they may never transition to the next wealth level either, and thus continue to leak this income stream forever.