or, I don't know, in case we do?

I think one reason why people are so touchy about any criticism or MarkGemma is that they embody the quintessential average heterosexual relationship and marriage. The relationship dynamic that more than any other is held as sacred in our society. To the point that I don't think people even pay attention to what is being shown because the moment you say "these two people are married and here is the montage of their story" they automatically project all those conditioned preconceived beliefs about what the relationship must be by virtue of being a marriage.

Never mind the statistics on divorce demonstrating that marriage doesn't automatically = some grand epic love. There is a reason half of relationships/marriages don't last and that is because they are often just build upon the motions of meet -> attraction -> date -> relationship -> marriage -> children (maybe), and don't withstand the impact with reality once it does hit. May the reality be life's hardships or discovering sides of the other person that the person smothered or downplayed in order to please and appeal to the partner during earlier stages in the relationship.

Which is exactly what we see in this episode. Like OP said, there's never any grand moment that makes you go "yes! these two are so made for each other!". It's all bland stuff that could be applied to any generic straight marriage out there. Granted, I don't think that you need one grand moment to cement a relationship as true love. I think this can be built upon small, every-day gestures, and by enduring difficulties and working through them together. I would go as far as to say that it isn't true that to tell a good story you necessarily need those epic romances; that there can be a good story in an everyday romance too. Everyday love, everyday pain, everyday struggles, can be compelling. To paraphrase what Dan said about what he likes about Markgemma, choosing to stick together when things get tough can make for a good story.

But that is kind of the point: sticking together is very different from growing together, or working things out together. Many people choose to 'stick together' and live unfulfilled marriages for the rest of their lives. And that is where their relationship was headed in this episode: Mark and Gemma weren't working through anything together. They were sticking together on paper while drifting apart on an emotional and human level.

One thing in particular that I find very off-putting about their dynamic is how as soon as things get difficult, Gemma is the one pining for attention and love from a distracted and uninterested Mark. Yes, she calls him out once about being an asshole, but for the most part she is just there waiting for his crumbs, and having to remind him to show her the affection she is showing him.

Having said that, and here is where I disagree with OP, I don't think Mark and Helly's is real love and Mark and Gemma's isn't. I think they both are real love, just different kinds of real love.

I think Mark and Helly's is the one that is build to last from the get go, simply because it is based on something that is just part of who they are as people that makes them not just compatible in terms of what they have in common (like their morbid sense of humour), but in terms of how their personality traits fit with the other like pieces of a puzzle and fill in the gaps the other is lacking: Mark is avoidant, but Helly is incredibly confrontational and doesn't let him get away with ignoring problems. She is IN HIS FACE like a dog with a bone whenever something is troubling them. And Helly is impulsive and destructive in that impulsiveness, but Mark is protective and knows how and when to throw water on that fire. He pays attention and is IN HER FACE whenever she tries to burn shit to the ground. What one needs to thrive is what the other is naturally inclined to provide.

Mark and Gemma's is the kind that isn't built to last from the get go, because its foundations are not based on nature, they're based on nurture - in this case the nurturing of society and the steps the average relationships goes through, with all the expectations that go with it. They are not innately made for each other, they have to work at it. It's the kind of love that can only last if both parties put an awful lot of work into it, because they don't complement each other in a way that comes natural to them. So, in a sense, this ordeal with Lumon might very well be the kick in the butt Mark and Gemma needed to sort their shit out. That it might make Mark realise what he had and took for granted, and truly make a change. It might make Gemma realise that she needs to hit him with a chair across the head when he acts like an asshole.

BUT.. and here comes the biggest but of all..

Mark created a version of himself that does not remember Gemma, never loved her, and now not only desperately loves someone else, not only is that someone the consciousness of the heir to the company that kept her in a basement for two years, but he has that "built do last" relationship with her. A relationship that is going through massive ordeals and is only growing stronger as a result. So now Mark might want to make all the changes in the world to be a better husband to Gemma, but not only he needs to put all the awful lot of work I mentioned above, but he needs to do it against the pull of THAT kind of relationship.