Wrote this with the ChatGPT prompt "Write Python code to send email using GMail API and a Service Account". Looks accurate though I've not tested.

gmailer.py

import os
import base64
import google.auth
from google.oauth2 import service_account
from googleapiclient.discovery import build
from googleapiclient.errors import HttpError
from email.mime.text import MIMEText
from email.mime.multipart import MIMEMultipart
from email.mime.image import MIMEImage
from email.mime.audio import MIMEAudio

# Define the email parameters
sender = 'your-email@gmail.com'
to = 'recipient-email@gmail.com'
subject = 'Test email'
body = 'This is a test email from GMail API'
attachments = ['file1.png', 'file2.mp3']

# Create the credentials object using the Service Account key file
credentials = service_account.Credentials.from_service_account_file(
    'path/to/service-account-key.json',
    scopes=['https://www.googleapis.com/auth/gmail.compose'])

# Build the GMail API client
gmail_service = build('gmail', 'v1', credentials=credentials)

# Create the message object
message = MIMEMultipart()
message['to'] = to
message['subject'] = subject

# Add the body of the email
text = MIMEText(body, 'html')
message.attach(text)

# Add the attachments
for attachment in attachments:
    with open(attachment, 'rb') as file:
        file_data = file.read()
    if attachment.endswith('.png'):
        attachment_type = 'png'
    elif attachment.endswith('.mp3'):
        attachment_type = 'audio/mpeg'
    else:
        attachment_type = 'application/octet-stream'
    attachment_name = os.path.basename(attachment)
    attachment = MIMEAudio(file_data, _subtype=attachment_type)
    attachment.add_header('Content-Disposition', 'attachment', filename=attachment_name)
    message.attach(attachment)

# Encode the message in base64 format
raw_message = base64.urlsafe_b64encode(message.as_bytes()).decode()

# Send the email
try:
    message = gmail_service.users().messages().send(userId=sender, body={'raw': raw_message}).execute()
    print(F'Sent message to {to} Message Id: {message["id"]}')
except HttpError as error:
    print(F'An error occurred: {error}')
    message = None