Deka means Detective, so I guess the title is an attempted parody of the 1982 TV series Space Sherriff Gavan, though that parody only goes as far as the title and the fact that the player characters transform into Metal Heroes-style armoured forms. Which makes this the third Metal Heroes-inspired PC Engine single plane beat em up I've covered on this blog. What a weirdly specific subgenre! As single plane beat em ups go, it's pretty basic. The stages are just completely flat planes, and you go from left to right beating up enemies until you get to the end of them. You've got a little three-hit combo, plus jumping attacks and a couple of throws. Beneath your health bar, you've also got a power bar, though! It's for managing your transformation! You can transform at any time, and the meter slowly increases while you're not transformed, and slowly depletes while you are.
The game's biggest problem is that the transformation is so limp. You don't get any new attacks or abilities, even the animations for your attacks while transformed are traced over the sprites for your untransformed attacks. It's so unimaginative! I guess you have higher attack and defence while transformed, but the difference is so incremental that you'll barely even notice. It might even be a placebo and there's no difference at all! Considering that it's around this gimmick that the game is built, it really sucks the joy out of the whole thing.
There's a few other things I want to talk about, though: in its favour, it is one of very few PC Engine games with a two player co-op mode, and it's an early example of a beat em up with a versus mode that lets you play as a few of the enemies, too. And on the subject of enemies (as well as returning to the subject of parodies), some of them are from other games. Games from other companies, so they're unofficial references and/or parodies, not guest stars. There's a pair of enemies who are literally just Ninja and Kunoichi from The Ninja Warriors, and a little later there's an overweight version of Guile from Street Fighter II. As well as being fat, there is an actual joke in the Guile parody: he has an attack where he attempts a FLash Kick, but falls over onto his back afterwards. It's not a funny joke, but it is something, at least. (Also, the NW parodies first appear in stage 2. They don't appear in stage 3, though, so when they returned in stage 4, I thought to myself "The Ninja Warriors, Again?")(Now that's what I call attempted comedy!)
I can't really recommend Gayvan, unless you're really really desperate to play a co-op beat em up on PC Engine. If you really want a Metal Heroes-inspired game and you don't need a second player to be involved, then just go for Cyber Cross - Busou Keiji. That game is excellent, and if you're buying real copies, it's like a tenth of the price of Galaxy Deka Gayvan. This is an inoffensive game, but it's also an unexciting one with so much wasted potential. To make it worse, it's from Fill-In-Cafe, who'd later go on to make a bunch of really great fighting games and beat em ups.