What would it takes for GOG to win against Steam ?

Steam currently has a monopoly on PC market, so not gonna happen under any circumstances, for now atleast, simple as that

"What would it takes for GOG to win against Steam ?"
Getting the support from Activision, Square-Enix, Sega and Warner.

Ignoring the Blizzard part of Activision-Blizzard, those are the bigger publishers who (as far as I know) rely exclusively on steam for DRM / digital distribution.
If they give up the exclusitivy, that would be something that could really undermine Steam's monopoly position.

You go to GOG for DRM-Free, solid customer service, constant upgrades and shifts to site procedures to cater to their customer base. GOG has built a brand around catering to players as much as possible.

Steam, on the other hand, is notorious for its horrible customer service, limited customer support [in certain circumstances], updating itself very slowly, and not listening to its customers. [Nevermind that the blue-on-blue of their website is nearly impossible for brain-damaged people such as myself to navigate, causing notable neurological pain to browse, something they're aware of, and willingly choose not to address. Which is akin to having a strobe effect on your front page that flashes out 'screw epileptics'.]
That being said, they've made good forward progress the past few years- vastly improving store layout and functionality, adding the recent generous game refunding, and you can't knock what a major boon bundles and Greenlight has been for getting people who weren't interested in Steam before, interested in it.

Despite having had an account from the very start of Steam [about 12 years ago], I didn't get involved with actually collecting games on it till a year ago. It was simply too stressful a client to use until recently.

But now?
Well, there are things that I still don't like about Steam, but then again, there are things I love: The complete control you have over sorting your game library, for one.

Fact is, GOG, Origin, and Steam all hit different corners of the market, and provide different benefits- and each have their own unique shortcomings.

Neither of the three is likely to simply vanish any time soon.

Noone likes Desura, though, and for good reason. :P

A better client would help, but to be fair they are working on it.

They need a bit more marketing. They just don't have the brand awareness of Steam.

GOG > Steam

The Witcher 3 GoG Inventory with Trading ability

And cards

And hats.

A Miracle ?

GOG is not trying to "win over Steam." They are building their platform the way they want it to be. Bothe GOG and Steam borrow ideas from each other, and that's all good for the consumers.

Time travel. Being the first helps a lot.

I only use Steam. I like all my games at the same place.

In a perfect world, if itch.io releases some "Galaxy" thing like gog, they should win because it's better for everyone.

gog still rejects most indie devs and also take a 30% cut.

But, if I have to choose, maybe I can predict now that gog might win in 4-5 years if they add their own virtual currency and community features.

Well, I ain't switching over to GOG until their user interface becomes more intuitive, have a more vast and updated content library and most of all, some way to import my existing steam library or atleast an incentive for me to 'abandon' the games I already have because lets face it, some of us do not want to use two platforms at once (i.e. Steam, Origin, Uplay) nor let go of our current library.

To me it's like vinyl records, cds, and tapes versus mp3s:Some of us will always prefer to own physical copies of music because it is ours and no one can just take every physical copy of my music unless they literally come and rob me. GoG's policy of DRM-Free means you own it, the company cannot arbitrarily take it back, it's that simple. Much like buying physical albums, you tend to pay more for the service than you do if you download mp3s or actually buy stuff on itunes, etc. Likewise, I have more games on Steam because it's cheaper and bundles are ridiculously easy to come by, so even though there's a risk that game companies or Valve could revoke a game from my library (which has happened), I find many other aspects of Steam to appreciate and make using it worthwhile.

a time machine

Free games.