If I am wearing a gradual tan moisturiser, can I still tan naturally from the sun?

You will still tan after having a gradual tan, the DHA is the active ingredient in self tanning products. This when applied to the skin will cause a chemical reaction to the amino acids present in the skin cells found in the top layer of the epidermis (stratum corneum). This causes a temporary darkening of the skin and does not change the way the skin functions in any other way.
This is why it's important to remember to tan safely and reapply your sunscreen. It's a good idea to use a higher SPF for your face and décolleté as this needs more protection.
Even though its a gradual tan if you start using it consistently you need to start maintaining your skin in the way you would if you had a spray tan or a regular self tan product. So your going to need to exfoliate and moisturise your skin frequently.
I hope that helps!

Hello - Yes, you can still tan and still burn, you should use an SPF as normal, to protect your skin from UVA & UVB rays. The active tanning ingredient, DHA merely dyes the top layer of the skin, it does not change any other function.

Yes, unless it contains an SPF of 15 or above.