Google uses HTTPS for their services, so the connection between your browser and the Gmail webserver is encrypted.
However, your company might be using a proxy server that is configured to intercept such connections, decrypt them, and reencrypt them with a company certificate - this is done to inspect the traffic for malware (for example to stop you from downloading that virus someone emailed to your personal account). In that case, the company would be able to see all the web traffic, and therefore could see all the emails you _open_ (but not all emails in your mailbox).
However, your company might be using a proxy server that is configured to intercept such connections, decrypt them, and reencrypt them with a company certificate - this is done to inspect the traffic for malware (for example to stop you from downloading that virus someone emailed to your personal account). In that case, the company would be able to see all the web traffic, and therefore could see all the emails you _open_ (but not all emails in your mailbox).