Hi Zoe
Those are appogiaturas rather than grace notes in the modern sense. As in Baroque performance practice, they should be played on the beat, take roughly half the value of the “main” note, and often slightly emphasized to make the most of the dissonance and resolution.
For an excellent overview of harp performance practice of the late 18th century, get a copy of Mike Parker’s book “Child of Pure Harmony”. In it, he brings together a lot of information from the many harp method books published during Krumpholtz’s time.
Hope this helps,
Paul