Developing stretch marks from gym workouts is very common and harmless, resulting from rapid muscle growth. For newer, red or purple marks, topical treatments are most effective. You can look for creams containing hyaluronic acid or Centella asiatica extract, which improve skin elasticity. A prescription tretinoin cream (a retinoid) is also highly effective at rebuilding collagen, though it may cause mild dryness. For daily hydration, massaging the area with sweet almond oil or rosehip oil helps. Please note that while these treatments fade newer marks, older white marks are permanent but will naturally fade and become much less noticeable over time.