Just to enlarge on what Browd has said, if you work through all the policies in the Liberty tree you can select a Great Person of your choice- including a GP. This is often a decent option if you aren't making much faith and it seems you will miss out on a religion. (I normally start out on this path anyway so if that is your style it is a little easier).
Agree. However, if Hagia Sophia is available when you complete Liberty, instead of selecting a Great Prophet as your "free" Great Person, you should strongly consider taking a Great Engineer and bulbing Hagia Sophia - you get a Great Prophet with HS, plus 3 faith and a free (and maintenance free) temple for 2 more faith (the 5 extra faith will boost you towards the second Great Prophet you need to enhance), plus 1 Great Artist point. Beats getting just a free Great Prophet every day of the week.