This issue started happening in eM Client 7.0 when we changed the way we work with Gmail. Since we started supporting Gmail labels and folder structure, all labels are synchronized.
And one thing that was not visible before (and might not be visible in other clients) is that Gmail assigns the 'Important' label to all messages synchronized through IMAP.

In eM Client 8.1 we added a workaround in the Menu > Accounts section under the IMAP tab.

Under the Tag section you will find the Show Important tags option and you can select to either show it based on Gmail's settings, Always show it or to Never show it. 

LEGACY OPTIONS

In older versions, you need to go to your Gmail settings in your web browser and create a filter that will remove the Important label automatically from each message.

To achieve this, go to Gmail webmail, open Settings > Filters and Blocked addresses. Create a new filter where all messages received by your email address will never be marked as Important.

Screenshots of such filter setting can be found below:

Note: You can either apply the filter directly to all previous conversations or open the Important folder/label and manually select all messages to remove the Important label