To me the image looks really fishy.
The glare around the UFO doesn't make sense, as I'd expect it to be centered on the bright portion of the object (its upper left), rather roughly the center of the object as a whole. Additionally the gridding pattern (old scanner artifacts?) can be seen clearly on all parts of the image (other than those parts that are essentially pure black), except for the UFO. This give the impression to me that the object was added in on top of that part of the image.
There are also some fine details of the UFO (a row of very short vertical stripes, almost dots) at roughly the mid point of its height, that seem a lot sharper than anything else in the image, which is a bit suspect.
Having had a quick look at NASA's gallery of Gemini photos (
tothemoon.im-ldi.com/gallery/gemini), I found that the images taken by all camera types, were square, not rectangular like the image in the OP, so it's at the very least been non-uniformly cropped, if it's from a Gemini mission.
Additionaly, there are very view Gemini photos that appear black and White, with those that are, being some of those taken with the Zeiss Ikon Contarex and none of those B&W images show the limb of the earth.
My guess is that someone has used an old image of the earth from orbit, from something other than a Gemini mission (might not even be from a manned mission), that had something in it where the UFO is in this version of the image. That something (perhaps a detached piece of the vehicle, such part of as a fairing) was creating a roughly even, oval shaped area of glare and the UFO was edited in over it. That's just conjecture though, as without the unaltered image it would be difficult to confirm that.
Edit: Just seen Mick's post above, with the original photo (he posted while I was typing up my comment), so ignore my theory of the UFO being edited in on top of some something else producing glare in the original image. I guess whoever made it, was willing to put in the few extra minutes required, to add that bit of patterned glare.