Pray tell, Pescador, what in the world is a Gaki? Have any images? What's the possibility that there's a hidden nail or two? A magnet or, better yet, a compass might be a diagnostic aid.
All it takes to hold it on is one little fiber. Look in stew-Mac catalog and find the neck remover & make one. Or you could turn the top down on a table and put 2x2x 34 nad suport the top on the table along side of the fingerboard and give the heal of the neck a sharp rap down. Of cource pad it with another block of wood.
Is the fingerboard all loose? I would first run a putty knife between the guitar top and fingerboard all around to see if that is holding. It will come off!
Welcome to the world of Yamaha neck resets; this is very common with Yamahas. What usually happens, is that a piece of the dovetail tenon has broken off, and remains glued to the wall of the socket. You'll need some sort of removal persuader, and you'll need to wiggle it as you're applying pressure. Don't move the neck too much from side to side, you can crack the neckblock--don't ask how I know!
Yamahas are a beast, to pull necks, but they are almost always worth the trouble. I convert them to bolt-ons, w/hanger bolts--if you want, I can post or email some photos of the process.
I think that the OP means, "Gakki", as in "Nippon Gakki", a 60s or early 70s Japanese-made guitar. Fortunately, they don't have the thick finish of the later ones, which is a whole 'nother can of worms.
I do have a neck persuader that I built, a copy of the Bryan Gallup jig..Everything is loosy goosy..I guess I'll go back to it..It didn't want to work the first time, but I didn't want to push it too hard..I would like to see the bolt on conversion tho'..
I just reset a Yamaha neck that had the heel broken at the dowel joint like the picture above...that was a heap of fun. Once everything finally came apart, the dovetail had left it's edges on the neck block. They used so much glue like it was going out of style. The pocket behind the dovetail was practically filled full of it.
I've sawed a few off, with my Fein Multimaster & jewelers' slotting blades, and a .008 thick pullsaw from Japan Woodworker.
These Yamaha dovetails are tight, tight, tight--I've never seen one with shims.
I just did a Blueridge, with a 5 dowel butt joint; I heated the joint with a heatgun, worried it back and forth until I got movement, then clamped the guitar facedown, to open the joint, and went at it with the pullsaw.
O.K....I've just thought on this....It can't be the dowel....It will move from the back to top of the guitar a good 3/16ths....It moves from the body out the same amount....So it all moves, it just don't come off yet...HMMMMMMM??????????
You say there is movement, so can you get steam in there? I also think that a section of the dovetail tenon is still glued to the wall of the mortise, and needs to be steamed out.