It is called Project Pilgrim - with £4m of the funding coming from the Heritage Lottery Fund - its aim is to improve the medieval building's facilities.
The proposals include building a new glass entrance, replacing the car park with a public space and installing solar panels on the Nave roof.
Project Pilgrim manager Anne Cranston said it was to "make sure the building is here for future generations".
Ms Cranston said: "From the city's point of view, there's so much regeneration going on in Gloucester and the cathedral is such an important tourist destination, we need to really work out what we can do to help with that."
She said the first thing people would see on site would be archaeologists to "reassure ourselves we're not going to disturb anything vitally significant".
"That's a very deliberate step on our part so that we don't hold up the rest of the construction programme. That'll just end up adding to the cost if we end up having to get the builders sitting there twiddling their thumbs," she said.
She said the "expensive" project had been funded completely by grants - as well as the Heritage Lottery Fund, other organisations included the Friends of Gloucester Cathedral, the Somerfield Trust and many individuals.
The project is expected to take three years to complete.
A £6m project to renovate Gloucester Cathedral has begun.