I'm certain I'm not the only one missing GOGMixes, and since there's also no way of searching for rail shooters through tags or what-not, here's an admittedly short list of said game genre on GOG:

Panzer Dragoon: Remake:
Absolutely brilliant rail shooter made with gamepads in mind, as it was originally made for the Sega Saturn. The remake is very faithful, and a lot of fun. Highly recommended, if you're into rail shooters. High replay value.

Star Wars: Rebel Assault 1 + 2:
Rebel Assault 2 is where it's at. They're not great by any stretch, but they're fun if you're into rail shooters and a Star Wars fan. They were made with a joystick in mind, but I think they can be made to work with modern gamepads. The sequel is better than the original, but both of them don't have much replay value.

Wetlands:
Rotoscoped animation, pulpy sci-fi story and cheesy dialog! Highly enjoyable rail shooter if you're into any of those things. It's better played with a gamepad, which has to be activated through the options outside the game, through Dosbox, yada, yada, yada. Takes some tinkering, and you'll need it, since there's a drone mission that's unplayable using the mouse (I'm assuming the game was developed with joysticks in mind). Its replay value depends on how much you like the setting and the presentation.

Cyberia & Cyberia 2:
I've never played this series, even though I'd seen it in magazines back in the day. Reviews were mixed, save for the graphics, which were considered top-notch back in the day.

House of the Dead, The:
Absolutely loved this one back in the arcades. I used to play this series with a friend, so it was more fun than it was supposed to be. I haven't played the remake, yet, but will eventually get to it. This one and Operation Wolf Returns: First Mission are the only games that were played using a light-gun back in the day, so I don't know how they fare when using mouse controls.

Operation Wolf Returns: First Mission:
Another rail shooter from the arcade days. I remember it had a chunky SMG type of light-gun. It was fun at the time, but I don't know how it translates to mouse controls since I haven't played it.

Honorable mention:
Space Harrier can be played in some of the Yakuza games. Although simplistic by later standards, it's still a lot of fun for what it is. Plus, you get a mobster brawler game set in Japan when you buy Space Harrier. :P

Cheers, much appreciated.

Actually thought of doing such a GOGmix ersatz thread myself for quite a while, compiling games on GOG that feature tank controls, and with the quintessential tank control games now being available it'd be about time, but I somehow still don't get around to do it.
Maybe because I also naively retain a sliver of hope that the actual GOGmixes might come back.

Cool idea for a list, and good catch with the Space Harrier.

The more forum GOGMixes the better. In fact, if we start this going, then maybe we can have a sticky thread collecting all those GOGMixes.

Or maybe GoG should just bring the damn GOGMixes back already!