Compresses/uncompresses a file using gzip compression.
When compressing (uncompressing), the new filename has suffix
appended (removed), which is also used to test if a file is gzip'ed
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Arguments passed to "R.utils::gzip" and "R.utils::gunzip", respectively.
Returns (invisibly) the updated pathname. When compressing (uncompressing), an exception is thrown if the file is already compressed (not compressed). An exception is thrown if the file does not exist.