The best alternatives to Gradient Zoo are Web Gradients, Colornet, and FlickFeed. If these 3 options don't work for you, we've listed a few more alternatives below.
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WebGradients is a free collection of 180 linear gradients that you can use as content backdrops in any part of your website. Easy copy CSS3 crossbrowser code and use it in a moment! We've also prepared aPNG version of each gradient. As a bonus, there are packs for Sketch & Photoshop.