It is and it isn't. Using the mutability of values in Go, like in C, is based on the assignment of pointers as needed, making it optional in some sense.
And now due to the inclusion of generics and first class functions, it is possible to use Go in a mostly functional manner much like Python.
So while it may be incorrect to claim that Go is a capital "F" Functional language, it is correct to say that it can be used to write code in a functional paradigm.
(It’s going to turn out there are as many definitions of FP as there are of OOP, isn’t it?)