First find the total number of minutes Gretchen spends sitting at her desk: 6 hours * 60 minutes/hour = <<6*60=360>>360 minutes
Then divide this amount by 90 minutes to find how many walking breaks Gretchen takes: 360 minutes / 90 minutes/break = <<360/90=4>>4 breaks
Then multiply the number of breaks by the length of each break to find how long Gretchen spends walking: 4 breaks * 10 minutes/break = <<4*10=40>>40 minutes
#### 40