Gunhound EX, which is co developed by G.Rev which also co developed Ikaruga and Gradius V with Treasure, has the same publisher as Astebreed, La Mulana and D4 which are all available on GoG.com.

I hope there are chances to get this game here.
http://www.gog.com/wishlist/games/armored_hunter_gunhound_ex

I got an answer from H2 Interactive which is the publisher of Raiden IV: Overkill. They released Raiden III on GoG.com but Raiden IV is supposed to be much better.

Greetings from H2 Interactive,

First of all, thank you for your interest in our game.
We have not confirmed our plans to release Raiden IV: OverKill on GoG.com yet.
As soon as we know, we will make the announcement.

Thank you for your patience and understanding.

Voted.

Since Ikaruga is probably the most famous shmup I hope you dont mind that I use this thread to get attention for other shmups as well.

Degica (they already released Crimzon Clover on GoG), has two more games on Steam that I would love to see here. Please vote for and [url=http://www.gog.com/wishlist/games/dzeal_deltazeal]ΔZEAL (DeltaZEAL).

Yeah! Vote for me! (?)

On a serious note, it'd be great. I also would like to see Mushihimesama Futari on PC, and a western release for the Touhou series.

Cave is basically dead at this point.

Though actually, Playism released Touhou 14 and some of the spinoff games in their store recently. And several spinoffs got released on PSN for PS4/Vita in japan - wonder if someone would be interested in localizing those too...?

So many quality shmups on Steam lately... We gotta beat that. Voted for all.

Voted.

I´m not very fan of this genre, but this one lokks interesting, voted.

Still haven't played this one somehow. I've been curious about it for years. Voted.

Ikaruga is a great shmup. It is always in the top ten of the Top 25 Shmups of all time for many years already.

So far from this list there is only Crimzon Clover and Raiden Fighters (part of the poorly ported Raiden Legacy) on GoG but most them never got released on PC.