Writing text to gzip file

mode="wb"

When writing to a file opened in binary mode, you must write bytes, not string. Encode your string using str.encode:

with gzip.open('file.gz', 'wb') as f:
    f.write('Hello world!'.encode())

mode="wt"

(found by OP) Alternatively, you can write strings to your file when you open it in the wt (explicit text) mode:

with gzip.open('file.gz', 'wt') as f:
    f.write('Hello world!')

The documentation has a couple of handy examples on usage.