Can an icon use a gradient?

Yes, the petrol gradient, on white, sand and light petrol grounds, and never on light orange or petrol; no other gradient is used on icons.

An icon needs 3:1 against its ground, and a gradient is only as strong as its weakest stop. Petrol's light stop clears that on white, sand and light petrol and falls below it on light orange; on petrol the dark stop disappears.