The previous answer is no longer the correct one. In the Kalos/Hoenn GTS, Alola GTS, and HOME GTS, you are able to search for Pokemon you have not seen before by using the "What Pokemon?" option at the bottom.
This can present problems, as people are able to request impossible trades, like a Ditto with a gender, or Pokemon below the levels they can legally be obtained as. However, you no longer need to have caught or even seen the Pokemon you want to search for on GTS.
The HOME GTS impossible requests were sorted out in an update, but all impossible requests made before that time stayed. The other two GTSs still have the impossible request issue.