Because you have a complex gradient, the design is more complex to create. However, you could try this method:
Create one gradient tile, for example:
Select the tile and Object > Transform > Reflect. Reflect horizontal and click Copy. Align the two tiles.
Select both tiles and Object > Transform > Reflect. This time choose Vertical and click Copy. Align the tiles.
Select all 4 tiles and choose Object > Pattern > Make. Click OK to add the pattern to Swatches.
If necessary, adjust the width and height so that the pattern repeats smoothly.
Then click Done in the upper left.
Your pattern will appear in the Swatches panel and you can use it to fill any shape that you create.
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