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Assuming that I am connected to the internet using a VPN service (say Hotspot shield or CyberGhost) and that I am browsing a HTTPS website (say https://www.gmail.com), can my VPN provider see the content of those websites?
No. One of the end is your computer, not the VPN provider, so the connection is encrypted between you and Gmail.
The VPN just acts as an intermediary at the IP level, but HTTPS is handled at the application level.