Gel filters quality

I believe that I once read that Gel filters were as high optical quality as single or multi coated glass filters, because they are so thin that they don't have the same effect on light as glass filters. I remember that sometime back then (early '80s) I believed it, but it doesn't make sense to me now. I've looked online & in “Basic Photographic Materials & Processes” for several hours & I can neither find confirmation nor denial.
Any input welcomed, Jay Drew

Kodak Wratten gels have excellent optical quality. I used them for years, and still prefer them. However, they are easily damaged and now very expensive to replace. So, today, I use B+W F-Pro or Heliopan SH-PMC filters which are durable and will last for years if properly cared for.