One option is to not go for that
exact keycap. The GK61 comes with what looks like cherry-profile keycaps, so the one you have a picture of would stick out farther and look kind of odd because it's SP SA R3.
If you were dead-set on that exact keycap, you may be out of luck because it was a GB organized by a defunct company that ordered the keys through SP almost 5 years ago iirc. There was a run of them on massdrop so theoretically you can request them again through the platform but there's little chance it'll ever be run again. However, GMK Carbon r2 had a half-life lambda pack (pictured) so your best bet is to try and find someone who is willing to part ways with the cap you want from there, or to find someone selling the whole kit. Those lambda keycaps will look much better on your GK61 if it still has the stock keycaps.
If you want to try and find someone here on geekhack who has the exact on you pictured, you could ask a mod to move this to classifieds, where you might have some luck.
Here's the kit I was talking about:
