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Reproduced on 3.11:
>>> from gzip import GzipFile
>>> from io import BytesIO
>>> file = BytesIO()
>>> with GzipFile(fileobj=file, mode="wb") as z:
... z.write(b"data")
... 
4
>>> file.write(b"garbage")
7
>>> file.seek(0)
0
>>> GzipFile(fileobj=file).read()
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/Users/iritkatriel/src/cpython-654/Lib/gzip.py", line 301, in read
 return self._buffer.read(size)
 ^^^^^^^^^^^^^^^^^^^^^^^
 File "/Users/iritkatriel/src/cpython-654/Lib/_compression.py", line 118, in readall
 while data := self.read(sys.maxsize):
 ^^^^^^^^^^^^^^^^^^^^^^
 File "/Users/iritkatriel/src/cpython-654/Lib/gzip.py", line 499, in read
 if not self._read_gzip_header():
 ^^^^^^^^^^^^^^^^^^^^^^^^
 File "/Users/iritkatriel/src/cpython-654/Lib/gzip.py", line 468, in _read_gzip_header
 last_mtime = _read_gzip_header(self._fp)
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "/Users/iritkatriel/src/cpython-654/Lib/gzip.py", line 428, in _read_gzip_header
 raise BadGzipFile('Not a gzipped file (%r)' % magic)
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
gzip.BadGzipFile: Not a gzipped file (b'ga')