Epistemology is the study of what can be known and how, in a truly abstract sense, not so much tied to practices, but to foundations. It is an ordinary subfield of basic philosophy.
Philosophy of Science as a noted discipline is very new, arising after the Scientific Revolution in Europe. In modern study it is often merged with History of Science, following the dictum that, particularly in this case, 'history without philosophy is blind and philosophy without history is empty'. It is dedicated to how people actually do science in particular, often as contrasted with how science describes itself and recounts its own history, and is generally not about theoretical foundations.
Gnosiology is theoretically a broader term, but not so much any more. It was introduced by Orthodox Christians as form of traditional epistemology with a provision for revealed knowledge. This sense is sometimes used purposely today by commentators like Quakers who place high regard upon inspiration (or 'experiment' in the archaic form) as a form of knowledge. (But also because we just love to be quaint and quirky.)
The root moved from 'episteme', 'certain', related to 'ascertain', to 'gnosis', a more general term for 'knowing', because revealed knowledge can, almost by definition, not be 'ascertained' -- it must be validated in different ways, through pedigree or mystical experience.
Attempts have been made to revive it to apply to fields like aesthetics, which also lack the kind of standards that can be 'ascertained' but where things can still be known. But few Western writers adopted the term.
Evidently this move from Epistemology to Gnosiology stuck in the East and Gnosiology is the popular Greek term for Epistemology among Soviet and post-Soviet philosophical writers. This usage has drifted back to the West as a synonym and not as something distinct from Epistemology.