You can set the gradient on color, and lay it down. Make sure you desaturate the pic though. You can do that by ctrl + u.

The hard way is to desaturate it, then put the gradient you want down, then ctrl + f, click color, and bam. Be sure to adjust the contrast though, it's boring without color.

Another way is to desaturate it, get a brush and make the opacity like 10%, then color it on your own.

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