Using reCAPTCHA fallback service
LibreOffice Chinese forum reported reCAPTCHA has alternative site to provide the service:
According to this topic, if you redirect the site from google.com/recaptcha to recaptcha.net via browser extension, the reCAPTCHA will be loaded as expected. So there is a solution to make reCAPTCHA works without being affected by GFW. We can firstly load reCAPTCHA from Google, if Google doesn’t responce, try to connect recaptcha.ndt instead.