You will pay ZERO tax on the sale, as it was a GIFT from your mother to you.
However, it could be treated as an inheritance from your grandmother to you, with your mother basically renouncing the inheritance in your favor. This is a legal issue, so you best ask the attorney about that. If so, the tax BASIS for the property is the stepped-up Fair Market Value on the date of yor grandmother's death, in which case the costs of the sale will probably completely wipe out any capital gains from the sale. You may have to report the sale using Schedule D, but no taxes should be due.
I am a bit surprised this was not explained to you by the estate's tax consultant.