Additional info.

"TOP GUN" "No. 2" "No.3" that I mentiond is just the additional comment. No display nor voice of such in the game.

Let me answer your questions as I can.

>It has 16 projectors that need of course 16 images sources...so 16 laser disc players...and so 16 video laser discs with different footage on them ?!
yes. but as I know, there was no 16 variations in discs but 2 or so on. they were playing different "chapters".

>Does the GH-28 played the mission "Project Dragoon", like the version we play on the GT-6 ?
Yes, the story was "Project Dragoon", but there was some difference between that of GT-6"M8774D" and GH-28 "M8774E"/GM-16"M8774C".
M8774E and C was almost the same. I've heard that the 1'st version (1990, OSAKA)"M8774B" was different from "M8774E", but I don't know that one.

Differences:
1. No  "Destroy the air post" mission in M8774E.
2. 4 Major targets was located in the enemy mothership. One was the main reactor, as you know on GT-6, and other 3 fixed targets (I don't know what that was exactly). "Tower Cannons" grow out  inside the mothership.
3. No "Tower Cannons" on the enemy planet .
4. No mission failure due to loss of "shield".
5. No "lateral duct" burst through mission, and therefore, DRAGOON locates in just the center of  citcle of "Spark Bits".
5. "Sparkbits" (spell may be not correct) was not 6 but 16.
6. "Sparkbits" move from left to right and they go round only once. So if you are the only player, you "must" destroy all "Sparkbits" in only "one chance".
7. "Ending movie" was short. The last scene was the explosion of the enemy planet (if they failed, DRAGOON try to break out but vanished in the ray of "CANNON SEED", and "white out".
8. No detail1 score report. Only "ranking" (A to E, "A" was the best) and "TOP GUN" ("GOLD EMBLEM":stylized gold dragon or so on),
"No. 2" (SILVER EMBLEM), No.3 (BRONZE EMBLEM).

>Do you remember the price of a single play?
 500yen(GH-28)/300yen(GM-16) a game (and admission fare, sometimes).
There was "Passport" system supported in "Wonder Eggs" (except very early time of the park), it was 3,900 yen a day (includes admission) / 3,000 yen (night-time discount).

>GH-28 wasn't a 'free access' game ? I mean there wasn't any coins door like the GT-6 ? It was like a ride, in an attraction park ?!
Exactly right.

>If it's a kind of 'ride', does the time of the game was always the same ? I mean what happen if all the players don't play very well ? Game over after only few minutes ?
 1. Fail to destroy the enemy mother ship if players don't play well.
 2. If fail to destroy 16 "Sparkbits" within timelimit almost 20sec. , just game over that I mentioned above.

>Argh... Maybe do you have other pictures of the GH-28 for us ?
No , but my friends have some. I'll ask it.

Well, It's just simple.

Pos. 1 : Insert coin(s) to start enrty,  countdown automatically and launch DRAGOON.
Pos. 2: Insert coin(s) to start enrty,  press "START"(green) button to launch DRAGOON.
Pos. 3: press "START" button to start entry sequence, pull (or push) trigger to entry(activate "Blaster(s)), and press "START" again to launch.

 Hello from Japan. My fellows referred your forum to me. I'm an another GALAXIAN3 maniac.

 I've played GH-28 more than 1,000 times. I've once heard that The 1st player
played 6 times more. That was so amazing experience and I'm eager to play it again.

 If I'm correct, The diameter of largest circle in the figure(of GH-28), the cylindrical screen, was 12m diameter.
~