The womanising rocker - who has previously dated stars including Jennifer Aniston, Cameron Diaz and Jessica Simpson - wants to show his girlfriend of five months how serious he is about her so has been working hard on a special gift for her to open on Tuesday (25.12.12).
A source told The Sun newspaper: ''John has put some serious effort into his Christmas gift for Katy.
''Since they met he has been trying to prove he isn't the guy everyone says he is.
''This should definitely prove that - every little detail on the guitar shows how much he thinks of her.''
The 35-year-old musician has had the guitar covered in intricate designs, including Katy's initials and the date they first met.
It was recently claimed that John and 28-year-old Katy - who was previously married to Russell Brand - are ready to move in together and have been house-hunting in Los Angeles, despite reservations from their friends.
A source said: ''John and Katy have decided to give it a proper go and want to find a place in LA to share.
''But lots of people are willing them to fail. They row a bit and have quite a turbulent up-and-down relationship, despite it only being a few months old.
''Many of her friends are certain it will all end in tears.
''But that's brought them closer in a way. It's given them an 'us against the world' mentality.''
The couple - who had a brief split in October before getting back together - are smitten with each other and like to spend every moment they can together.
One source said: ''They are together virtually every day.
''They hang out at John's place, cooking, watching movies and playing music.
''They take the odd trip away but generally they're trying to keep it behind closed doors.''
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