Romantic Shorthand?

If they have to fight a common enemy early on, that can help a reader see their relationship as close from the start.

Esmae, could you provide a little additional info? At what point does the gaslight-split happen? Because to my mind, if you have an established couple, the word "established" indicates we already see them together in the story. If their togetherness doesn't establish the strength of their relationship, you'd have to short that up. If this gaslighting happens in chapter 1, and "established" is merely "they'd been happily together for 6 years" then it would be a different issue/solution. Understanding the timeline might lead to more meaningful assistance with item #2.

Is he too willing to go along with the gaslighting? Maybe it would help if he's somewhat resistant, at least at first? Without seeing anything, I would guess it might possibly be a matter of balance, where the husband is doing the nasty creature's dirty work, but the wife is not, thereby putting her in the role of victim and sympathetic?

Oh, wow. OK, you've got quite a hill to climb. 1/2 a book of him being turned against her is a lot to overcome, and "Not a hot mess" implies they're a bit of a mess, which introduces both relationship stress points, and natural skepticism to the reader. This conflict should probably be a solid foreshadowing of how they operate together, especially under duress, so your readers remember what they're like when there isn't an antagonist interfering. It sounds like you have to buy some benefit-of-doubt. (Sorry to be so wordy here. Skip to the bottom for the shorthand. )

Just spit-balling some ideas here, because it's going to depend on what concept you want to use to get them back together (I'm assuming you DO want them back together or her leaving him would be fine.) Evelyn's suggestion is a good one, but maybe instead of a physical enemy, the enemy is what is making them "not 100% happy." Say, for example, it's finances. You would show them confronting that together, him sacrificing something he really wants or possibly needs for her to have something she wants or possibly needs, or show him doing every small thing to demonstrate his feelings for her--picks wildflowers for her because he can't afford to buy them from a street vendor they see when out together, or whatever. I think he's definitely going to have to demonstrate some "sacrifice" on her behalf. Frex, not, "I'm not buying this video game because money" but rather, "I was going to buy [X game] I really wanted but I saw they had a used version of [Y game] and I know it's your all-time favorite so let's play!" Not that exact thing, of course, but that level of his mindfulness, which should build reader-trust in him. (And also build reader emotion when they see him being transformed by the antagonist.)

They could also work together on something, maybe something his sister does or says. Not *against* his sister (as they would an "enemy"--you don't want to sow distrust in them as an empathetic couple--but maybe they have to work together so they can to alleviate her issue. Even so small a thing as never leaving a dish in the sink. They cover each other.

Then, of course, there's the romance side. You don't have to have a sex scene, but stealing kisses, or him expressing his desire and attraction in a way that doesn't seem opportunistic or shallow. Or him noticing something only he would notice because they know each other intimately. (I don't mean physically intimately, although that could be a thing, too. I mean he knows her every mood and facial expression. Maybe she has a pre-menstrual migraine, and he can tell she's hurting just by the pattern of lines by her eyes or something.) And then show him *doing* something nice for her.

So, I guess, tl;dr would be

1) sacrifice on his part (since he's the one who will turn against the relationship)
2) show how they work together on a common battle
3) make him extremely likeable vis-à-vis his feelings for her

eta: Carrie's point about how he behaves during the gaslighting is important, too. He needs to struggle during that transformation. If you've laid the groundwork in the first 25% of how he responds to her normally (in positive ways), then you give glimpses of him questioning himself during the gaslighting that hearken back his natural tendencies in the early scenes, readers should be able to reset their feelings for him (if he hasn't done anything too horrible.)

Well the most effective way I've read for established couple usually is when the couple is introduced in a previous/another story. However, for the first time meeting the couple and it's the first/only story, I usually like it when there are flashbacks (ie. when they first met; first kiss; first time he or she knew the person was the one). Generally, any happy period flashback to help sell the love. Also clearly defining where it all went wrong -what was it that caused such a gap in their relationship to make the relationship so strained.

I know better the things that don't work for me ...like when the problem between the couple is a big misunderstanding which could be cleared up if they simply talked. Or throwing in sex scenes to resolve an argument.

Congratulations! You've created a formidable villain! If your villain's gaslighting campaign were easy to see through, then it would be easy for the heroine to see though, and she would be too stupid to live for ever falling for it, and it would be no great feat to finally overcome it.

So now you have your work cut out for you to make your heroes a match for your villain, and convince your readers as well as your heroine. Not sure any established basis for the relationship will do the job, as the past fades. The hero's actions within the story will speak louder than the villain's words.