Is there any way of marking an email as read/seen using Gmail's API? Link to the API below

I tried going over the the entire thing, read the Java client reference too, but it doesn't seem that there is a way to do it, which is strange as it is possible to mark an email as read using IMAP wth Gmail.

It seems it's only possible to add/remove labels from messages

Can anyone confirm this?

You can add/remove the UNREAD label and it will update the email status. It was already mentioned in the comments by joshmeranda and Linda Lawton - DaImTo but I wanted to provide a code as an example using Java.

This would update a message using a given messageId by swapping its status from READ to UNREAD or vice-versa:

        String user = "A_USER_ID"; //<- use "me" to use the authenticated user's ID.
        String messageId = "A_MESSAGE_ID";
        Message message = service.users().messages().get(user, messageId).execute();

        if (message.getLabelIds().contains("UNREAD")) {
            ModifyMessageRequest modifyRequest = new ModifyMessageRequest()
                    .setRemoveLabelIds(Arrays.asList(new String[] { "UNREAD" }));
            Message modifiedMessage = service.users().messages().modify(user, messageId, modifyRequest).execute();
        } else {
            ModifyMessageRequest modifyRequest = new ModifyMessageRequest()
                    .setAddLabelIds(Arrays.asList(new String[] { "UNREAD" }));
            Message modifiedMessage = service.users().messages().modify(user, messageId, modifyRequest).execute();
        }

Reference:

• Gmail API Java Quickstart to get the Gmail service object.