This goes into the topic of "offsets" and how sometimes the same sequence of numbers is defined differently depending on how it is conceptually used, eg, traditionally, the first Fibonacci term is Fib(0)=0, but in combinatorial problems that makes no sense so Fib(0)=1. 

School arithmetic progressions make life easy by letting the first term be a(1). But if, say, the sequence of odd numbers is defined by f(n)=2n+1, then it seems clear that the first term is f(0)=1.

Ultimately, it is a matter of context. It is also a matter of unresolved confusion that linguistic, mathematical and computing ordinals require that context to be unambiguously translated.

In this case, I hope it was clear that G(0)=a is the first term.