First, the indicator refers to GDP and we can justify this simply by what the author said "the indicator ITSELF".(notice "ITSELF")

Second, "a means to an end rather than the end itself" means that GDP, by seeking " alternative indicators of well being", did not solve the problem of how to compare between countries(it wasn't a means to an end), it compares other indicators of well being instead.

Another way to explain the definition of "a means to an end" is "a way to get there". For example, going to college can be seen as a means to an end, because you don't go to college simply to go to college, but to earn a degree. The degree is the "end", and going to college is the "means".

I hope this helps! Please let me know if any of this is confusing.