I have fixed that by replacing key decoding in two places in gauth.py
key = base64.b32decode(secret[0:8], casefold=True)
key = key + base64.b32decode(secret[8:16], casefold=True)
key = key + base64.b32decode(secret[16:24], casefold=True)
key = key + base64.b32decode(secret[24:32], casefold=True)
key = key + base64.b32decode(secret[32:40], casefold=True)
key = key + base64.b32decode(secret[40:48], casefold=True)
key = key + base64.b32decode(secret[48:56], casefold=True)
key = key + base64.b32decode(secret[56:64], casefold=True)
As longer key seems to be not supported by b32decode.