You are able at the moment to sort "greater than" or "less than" for any properties that have the "number" property type.
For example, if you go to add a filter for "recent deal amount" (which is a number-field type), then you get the ability to sort by is "greater than" or is "less than."
Check if the property you are using is a single-line text property, if so, currently, it is not possible to filter because these properties are text entries.
As a workaround, you could filter for the property you would like "is known", export those contacts into Excel, and sort from there. Or you could build a new property that is a "number" property type to allow you to sort within HubSpot.