How is the game treated by dev here overall? Is GOG getting all updates and fairly on time? Is it missing any content here? Are tere some features locked behind Steam? Is planned release at least roughly the same or will we have to wait weeks or perhaps months?
Anything else I should know?
I want to buy the game but I don't want to get inferior product and suppoert comapred to Steam customers.

If you happened to play Dead Cells, maybe you could let me know, is the story as slash your wrists emo, as the one from Rogue legacy? Because that's what I hated the most from Rogue Legacy.

Sort of, they share having persistent upgrades but Dead Cells has progressive levels with different branches to explore, while Rogue Legacy has one big level. I'd say that Dead Cells is an all-around much better game. The controls are more fluid and feel great to play with, the combat is much more fleshed out, the presentation is better and there's more variety to the gameplay. That last one is done by having different weapons to play with, more enemy variety to fight against and importantly more upgrade paths both across runs and over the course of them, which includes being able to upgrade your character through getting better gear, skills and level ups and not being limited to using what you start that run with.

To compare, in Rogue Legacy you pick your descendant, which gives you the regular sword swing, any special trait they have and their special ability. In Dead Cells, you start the run with two pieces of primary gear randomly picked from all the gear that you've unlocked up until that point, which includes axes, boots, shields, spears, daggers, bows, magic, whips, and more, and you'll quickly get two pieces of secondary gear that represent your skills, such as different traps, flashbangs, flash-step, turrets, vampirism or a grappling hook. All these pieces of gear, barring the stuff you start with, also get secondary traits that alter how they work, such as doing bleed damage, freezing surrounding enemies after a kill or having you do double damage while also receiving double damage, which is how you tweak your build in addition to the different types of level that you can get. Dead Cells does get harder than Rogue Legacy, but is more fun to play which makes restarting less of an issue. It's a great game for if you just want to pick up and play something.

So far the updates have been very timely, same day as on Steam or only a few days afterwards. GOG has all the same content and features, which includes the daily runs. You also don't need Galaxy to do the latter, all you need is to be on-line, unless you want to upload your score to the leaderboards. Nothing so far suggest that any of this will be different for the final release.

Story hasn't been introduced yet, but what is there is more towards the weird in tone than edginess - you're kind of a sapient pile of goo possessing a headless corpse, so it's doubtful that there'll be any familial drama. I don't think that it'll end up a game that you will be playing for the storyline however. The gameplay is what you'll want to get it for.

Uh? What do you mean?

SPOILERS: All the characters you play as, have a family that they abandon, enter the castle, and they die forever. Their children follow the same pattern, until one of them manages to kill the last boss, only to then become trapped inside the castle, and become the new boss.

EDIT: Also the soundtrack is disturbing, because it's mostly made up of creeks and cracks. At least I found it that way. I've written a huge review for myself, just in case I forget, in which I explained why I hated Rogue Legacy so much.

Looks like a bold update! Me like it!

Thank you.
That sounds good.
I will most likely get it here then but I admit I am still a bit afraid as new games here were a bit of hit and miss here on GOG so far.

As Fortuk already pointed out: You play as a head-sized lump on snot controlling a decapitated humanoid corpse.
So far there are interactions with 5(?) friendly characters in the game. In a world with somewhat morbid environments and monsters the player character is overall cheerful and positive during these interactions and is portrayed as a bit daft. My expectations for the final screen of the game involve a hint at a bleak, overarching story behind all this - which the player character promptly ignores in favor of a huge cake.
If you plan to pick up Dead Cells do it for the gameplay, rather than for the combined 4 minutes of story.

This is the level of story I picked up also, you're a magic pile of goo that that animate a body then explore until you die and try again in a new body

For me Darkest Dungeon has the perfect amount of story for a roguelite. It accommodates so much world building.
Thank you for your answers.

1. IMO there's nothing slash-your-wrist emo about Rogue Legacy. The only emo game on GOG I tried to play was the first Trails.

2. This sounds amazing. I am now 100% determined to persevere through the terrible precision jumping and play Dead Cells for the story and cake.

I was already quite decided to buy it it was more of question where. I watched few streams of it before and read and heard a lot of praise toward it, so quality of the game wasn't really issue for me.
I am not going to play the game before it is finished as I never do but I headr it could get price increase when released so it seemed like good oportunity to grab it.
I already bought it and did so here. As I said, I was afraid it won't get the same treatemnet here as on Steam but seems it is treated well and even if it was to change, I am willing to take the risk.
I think there is better chance devs will pay more attention to GOG and won't abandon their games here if it sells well so I have to help that to happen. :-)

Or they will think "Hey, if we can get away with paying zero attention to the GOG forums and still make decent sales, maybe we could get away with that on other marketplaces as well." Psychology is a tricky subject... (;
Either way, welcome to the club.