>>531549961
>>531550256
GALLERY looks like it could be hinting at a boss rush mode, since that's the name of the jingle that plays before you fight a main boss

>>530010756
The slashed angel head actually looks not unlike the crossed out figure in the GALLERY prophecy. Which seems to have something to do with the bosses (Gallery is the tune that plays right before a boss fight at the end of each chapter except 3), or something to do with Goners or the Egg Man (stone darkners in the room, slashed out man shape). Hell the negative space might even be a FRIEND shaped shadow in the background

https://www.youtube.com/watch?v=L0MAep7ml3A
https://www.youtube.com/watch?v=7X-ltLVW_sE&list=RD7X-ltLVW_sE&start_radio=1