Simply the gate phase process it's an iterative life cycle and it's have another name waterfall.

it's funny. I was wondering myself and googling this and when I read your question it hit me !!.
if you want to have a phase gate approach then it has to be consecutive as a phase gate is a way to consider whether to proceed with the project or not (Kill point). in Agile, the project is divided into separate iterations where team select a specific number from user stories in the backlog, and at the end of the iteration or sprint, they review the phase with stakeholders. Agile does not mean parallelism.

Hi, I found this question on Oliver F. Lehmann prep test (200 questions document):

123. You are using a phase gate approach for your project. What do you need to consider?123
o The order of phases must be strictly consecutive without overlapping in order to allow for gates.
o Gates are often called kill points, when they are used to eliminate weak team members.
o Phase gates are often used to verify entry criteria of a previous phase and exit criteria of the next one.
o Approaching phase gates early is a common signal that project planning has been poor.

Correct question is (A), I couldn't find that statement anywhere in the PMBOK, and I don't completely understand why that should be true. According to that, phase gate couldn't be used for agile approaches, where sequential-iteration based phases or continuous overlapping phases are used?

Thanks!
Julia