Hello i'm new here! So i realy think that Godric Gryffindor is in fact the half blood prince. JK said that the prince was someone that knew things about a distance past. So it could be him and may be harry could "travel in time" like he did with Tom Riddle diary's to see Godric and something that he could want to show! Maybe Godric has put some of his thougts and memoires in the Pensieve like Dumbledore do. And the Pensieve we can see on HBP cover is an important pensieve where Harry can see Godric in the past and important things.