I have a photo that I have greyscaled and wish to place it over a green gradiant to use on a website.  I have been all over the web/tutorials trying to find an answer...help please!

why not change that grayscaled into a cmyk or rgb mode? 
Check the settings of the green gradient layer image.. (image --> mode) change it into that..then layer (place it over your gradient) it in.
Changing a grayscale into a cmyk or rgb won't add colour.. it just changes the mode..

happy photoshopping..
cheers

Thanks for the suggestion...I'll give it a whirl tomorrow.

Cheers to you as well!

 Is the greyscale image just going to sit on top of the gradient? or are you going to fade the images into each other?

if it's just going to sit on top as a different layer in photoshop, then open both files in photoshop and just drag the greyscale image on to your gradient image. this will create a layer with your grey image leaving your color gradient intact. (if you were to do it in reverse, it would strip out the color in the gradient layer to match the greyscale color mode...)

as erosiaart stated, it only changes the color mode to match the destination files color mode.

hope this helps,
retrocity

If you want a B/W image over a gradient, an easy way is to set the B/W pic's layer to Multiply. Then have that layer right on top of the gradient layer. At least that works if what you're talking about is how I picture it.

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