I got pretty deep into this game the past few days. I posted thoughts in the "what are you playing now" thread, the gist of which were "awesome but super easy (on normal), barely an inconvenience." But there's a lot more to say about this one than a general purpose thread can/should contain.
Gate of Thunder was an all-star production. Hudson contracted Red Company, the talented hired guns behind a ton of classics from Bonk to Sakura Taisen to Gungrave to do it for them. Red Co. in turn subcontracted Tsujikawa Osamu, designer of Thunder Force II & III, who had recently poached Fukuda Izumi, chief programmer on Thunder Force III at Technosoft. Nobody involved was even remotely shy about this being Hudson doing a Thunder Force game; if they'd called this Thunder Force 3.5 nobody but Technosoft's lawyers would have batted an eyelash. The weapons, weapon system. speed controls, vibe, music style, almost everything feels lifted right out of the Thunder Force series.
Upon reflection, looking at how hard Hudson went with this one I kind of wonder if NEC were planning for this to be the pack-in for the NA release of the Duo during development. It released in Japan about 8 months ahead of the North American Duo launch and was a direct shot across the bow to Sega, whose Sega CD also launched in fall 1992, showing the Duo could beat the Genesis at its own best shooter at the time. If so it might explain the default difficulty, wanting it to be accessible to a general audience rather than scaring them off on the first stage.
Pictured: NEC/TTI's marketing department accurately dunking all over lame Sega CD pack-in Sol Feace:

It might also explain why they splashed out on expensive Westerners in SYN Sound Design for the music (not T's Music who did Lords of Thunder & Cotton, as many [including me before like 2 days ago] incorrectly assume). Nick Wood is credited with "sound design" in the credits, a Brit who started SYN in Tokyo in 1988 with Simon LeBon, lead singer of Duran Duran (SYN stands for Simon Yasmin Nick, Yasmin being LeBon's wife). I think it's fair to ask if LeBon was involved in the production of the soundtrack but left uncredited, because of how uncool video games - or commercial contract work in general for a name artist - were still perceived to be at the time (à la Michael Jackson on Sonic 3). I don't know if this particular question has ever been seriously explored elsewhere, but SYN did a ton of work for various Japanese and Western media in the intervening years.
Pictured: Hudson Soft contract temps acting like they're soooo much fancier than you:

I did find a Kotaku article from 3 years ago linking a Reddit post claiming LeBon did the music himself, along with another French site also making the same assertion, both with no evidence to back it up beyond LeBon's involvement with establishing SYN. So yeah, nothing credible, but also far from impossible to believe. Being a coked-up international 1980s rock star fashion icon requires a LOT of steady income to seriously maintain. Especially when it's the 1990s now, Come Undone hasn't come out yet and people aren't taking you so seriously anymore and you hate your keyboardist even though he's not-so-secretly the most important part of the band.
Pictured: Synthesizer nerd struggles for his very life against carnivorous retirement home decor:

- ANYWAY -
Back to the difficulty for a minute.
Hudson, in the midst of their Summer Caravan phase and quite plugged into what shmup fans wanted, may well have been self-conscious about the light default difficulty. I stumbled across a print ad they put out in Japanese magazines emphasizing the multiple difficulty levels:

The text here reads:
February 21st War Will Be Declared!!
There are three modes: Normal, Hard, and Devil. Even if you are confident in your skills, it might be safe to start from normal. The one having painful thoughts will be you! The enemy is strong! We pray for your battle.
Although it might also just be flat-out wrong, maybe they made Simon LeBon write this too.
The thing with this game is that hard absolutely feels like the 'real' game. Normal has a ton of empty sections, enemies barely shoot during most of the game, bosses only have one or two attacks, I think there might also be enemies that don't even appear. Hard modes at the time were often just making popcorn enemies tankier, bullets faster, or adding revenge bullets. Whereas hard and devil difficulties here are very deliberately arranged, bosses have attacks and phases you don't even see on normal, and bullet spreads are all over the place, not just an idle dot or two floating past in the early stages. Going back to normal feels like half the game was just deleted.
Devil, despite its intimidating name, isn't even all that bad. Because it retains the game's built-in player advantages, it grants a lot more leeway than an average arcade shooter would: shields, your first power-up always granting you options (unlike Thunder Force, where the claw power-ups are sparsely spread out, not even appearing on every stage), extends in every stage, and every pickup becoming a screen-clearing bomb if the related weapon is already maxed out. It's still pretty ruthless, but arcade players won't run into anything they can't handle.
Lords of Thunder is the game from this team that stuck in the popular imagination, overshadowing Gate to the degree that I was pretty surprised how deep I got into it. I was going hard at devil difficulty last night, started getting tired and checked the time thinking it might be 2 already, only to see that it was suddenly 5am. There's so much more to say about this game, but I'm sure I've already well exceeded the amount of text anyone wants to read from me. But I did want to put a serious thread together, because searching here most of the discussion is way too sporadic for a title of this calibre, not to mention from like 2005.