You should be fine.
If you read the general conditions of the GP model with Sanagate, you'll see under point number 4.2 that it says you can switch from GP model to normal for the beginning of the month following.
Again, it mentions in their general conditions that yes, you can change doctors, they have a special form online that you must use but it is possible.
Remember, Sanagate work with a list of doctors so your new doctor must be on their list.
Standard insurance is the same with all companies in terms of cover.
With the GP option, you have to visit your GP for every new ailment. If you saw your GP, who referred you to a hospital and you are now visiting the hospital for the same issue, then you should be fine.
If you had to name your GP at the beginning of the year and his/her name appears on your policy then you must have opted for the GP method.
You may feel like you're paying for your medical expenses as you have a high deductible. Is this the case?
Hope this helps