make a backup. make sure you have a copy of everything you want to save/keep on your computer. full wipe (twrp = data, system, dalvik, cache. cwm = wipe data, wipe cache, wipe dalvik, format /system). flash rom and gapps.
when you're coming from the stock firmware or a different rom, you have to perform a full wipe before flashing.
Use all yours methods. (Wipe issues) And still not able to boot up. Not the issue of gapps. Coz I only flash the purity rom, and reboot, but still stuck in the four color circle for over 15 mins.
Finally I decided to give up and return to stock ROM.
OMG! I though I was the only one facing this issue.
This has happend to me exactly as mentioned on the first post.
I was using purity Rom from 08/01 build & with every update I never had any problem But recently I decided to erase everything & do a factory Image procedure which obviously erases everything.
So after factory image I wanted to return to purity Rom & got stuck on booting logo for ever.
I followed the steps exactly the same as mentioned in above post.