The rear button goes through the larger central hole in the rear of the Bigsby-style trem on the centreline of the body.
The other button can either go on the treble side of the neck heel (like many acoustic guitars do), or on the back of the guitar so that the screw goes through the rear of the body and into the neck block (like on a Gibson ES335).
If you've got an inspection mirror, you could have a look at the internal construction to see if this idea is practical. Or else use your phone to try and take pictures of the inside.
The former keeps the guitar pulled in slightly more to the body, but may get in the way a bit if you like playing the upper frets a lot. Have you looked at the guitar gallery section of the forum? A couple of pictures of the rear of built GR-1SFs there and they've gone for the treble side of the neck heel option. https://www.buildyourownguitar.com.a...l=1#post110654
On a real Gretsch, the strap button would go on the side of the upper front bout, but that requires a wooden fixing block behind the body ply, which the kit doesn't have. However, it could be possible to add one yourself if you haven't glued the neck on yet. You'd need to be able to fashion a block of wood that would fit inside, sitting on top of the wood making up the neck pocket. If you can push it in with your hand, you should be able to use the strap button mounting screw to hold it in place whilst the glue dries. You'll probably need someone else to help with this (someone with small hands would be a bonus here). That part of the body is pretty flat, so as long as the block isn't too tall, then you shouldn't need to have to try and radius the front of the block.
Various strap button positions are shown here in this StewMac strap button position explanation. https://www.stewmac.com/How-To/Onlin...ap_button.html
Position 3 is the Gretsch position as shows what I tried to explain above with adding a block - but it's obviously harder with the body already constructed.