It is my understanding that in gastrulation epiblast cells migrate to the primitive streak where they start to form the three germ layers; does this mean that gastrulation only occurs at the caudal half of the developing embryo
It would help a little in answering your question to know more about what organisms you are interested in, or the specific source that you are referencing, with e.g. the relevant quote.
For example, the primitive streak is specific to amniotes: avians, mammals, and reptiles. In other common model organisms such as flies or amphibians gastrulation happens differently.
Another thing to keep in mind is that gastrulation is a very large and structurally distending process, so it can't be really said to happen "only on one end" of the embryo. The whole embryo is going through gastrulation. We can however talk about the placement of certain gross morphological structures on the embryo. But really each cell is affected by gastrulation, whether it's immediately obvious under a microscope or not.
Looking at the Amniote section of the wiki page on gastrulation, it appears that the main morphological changes in gastrulation do occur on the caudal side - for amniotes.
However, in Drosophila, gastrulation involves the formation of an approximately homologous structure called the ventral furrow, which is obviously on the ventral side, extending most of the anterior-posterior axis. However the invagination event does actually happen on the dorsal/caudal side.
So, it's complicated, and it depends a lot on which organism you're talking about.
Update following response
Humans are amniotes, they follow more or less the normal amniote path in gastrulation according to the wiki link above.
An important note missing in my previous answer above is that gastrulation itself defines bilateral symmetry in embryos; prior to this establishment animal-vegetal is the only obvious axis. So we are kind of defining the rostral-caudal axis according to the formation of such structures as the primitive streak, which are formed during gastrulation.
So it's a little circular to say that gastrulation happens on caudal end only; as gastrulation includes the step that establishes the rostral-caudal axis. Without gastrulation that axis just doesn't exist.
Again, gastrulation is a process that happens to the entire embryo. It doesn't just happen in one area. Structures such as the primitive streak are just obvious clues that the process is happening. It might be proper to say that the primitive streak begins forming caudally (I don't know of exceptions, though I'm not an expert), but it would not be proper to say that gastrulation is a caudal process.