One doesn't need to go to GR; SR is enough for the argument.
Eternalism (the block theory of time) states that the past, present and future exist.
This should be contrasted with Presentism which states that only the present exists.
To see what this means, consider as is normal in physics, time as a dimension in analogy to space; but whereas we have the physical ability to move however we wish in three dimensions of space, we cannot do that with time; I cannot walk into the past, and nor the future; and nor is there some putative time-machine that will allow me to do this; this motivates Presentism - we are always at the present moment of time; this also is consistent with our direct experience as opposed to how we think about space and time, especially theoretically.
In Classical Mechanics, with its Newtonian notion of Absolute Time we are granted absolute simultaneity: everywhere can have exactly the same time; it's a requirement - it must have, not it 'can have'; hence presentism is a possibility, as is Eternalism.
So we have two options for an ontology of time.
However, it's suggested by the Rietdjik-Putnam argument that this is not possible when Relativity is factored into this classical picture ie in SR; since every event at 'present' has its own plane of simultaneity, and these need not coincide. Hence Presentism is ruled out, leaving Eternalism.
Which probably means Presentists have some work to do ...
