WITH r AS (
SELECT *, area(geometry) as aeraInter FROM
(
    SELECT
        *,
        area(greyLayer.geometry) as areaGrey,
        Intersection(greyLayer.geometry, redLayer.geometry) as geometry,
        greyLayer.geometry AS geomGrey
        FROM grayLayer, redLayer
)
)

SELECT *, (aeraInterSum / areaGrey) * 100 AS overlap FROM (
    SELECT idGrey, aeraGray, sum(aeraInter) as aeraInterSum, geomGrey
      FROM r GROUP BY idGray)
  WHERE aeraInterSum / aeraGrey > 0.9