Gr*via becoming canon won't make it good.

Gray/Lyon ? Love it, especially with their shared backstory.

Gray/Cana ? Childhood friends to lover is my favourite trope.

Gray/Natsu ? Those 2 are ready to lay their lives down for eachother.

Gray/Erza ? Great.

Gray/Lucy ? I like to imagine them having playful flirts all the time.

So basically the short version of my question is: is Juvia saying she doesn’t deserve to love Gray specifically a form of gaslighting?

I didn’t know how to answer this question at first, because I knew it was gaslighting even if I wasn’t fully sure how to explain how, but after a lot of thought, I’ve figured out how to word it and how to explain why it hurts so much more than traditional gaslighting. 

You’re focusing on the wrong part of the scene, and arguably, the wrong character in the scene.

When she says that she doesn’t deserve to love Gray, it’s emotional manipulation and a guilt trip in order to make him more likely to tell her she’s wrong, and to think that he’s wrong to be more upset than her. She’s using it to aid in the gaslighting, but that part is not the gaslighting itself, at least not in this particular instance.

It becomes gaslighting when she uses it as a way to coerce him into forgiving her for the things that she says and does in the rest of the scene. She stalked him across several contries, and admitted to killing the necromancer that controlled his dad, but she only gave a half ass apology for the first one, which we never even see Gray accept. She starts to turn the situation into one where her own “guilt” is more important than Gray’s sadness, which we see as she piles the blame on herself and claims that she doesn’t deserve to love him. 

All of the emotional manipulation and guilt trips combined gaslight Gray into ignoring his own grief to comfort hers, because rather than her gaslighting him directly, she’s making him gaslight himself. 

(Gray should not be the one apologising in this scene. From every angle, Juvia's in the wrong here - killing his father, stalking him to Isvan, pouring salt on his grief. Gray apologising hurts so much, because it's an apology coming from the innocent party, not the guilty one.)

For example, if an abusive parent tells their kid who has chosen to speak out against them or to them “I guess I’m just a bad parent then.” It’s a guilt trip that’s being used to get the child to backtrack on their statement. They may bring up the good things they’ve done and turn it into a “woe is me” session to guilt trip the victim into thinking they were out of line, and that their hurt is lesser than the hurt their parent is feeling for being called out. This causes them to gaslight themself into apologizing or into comforting their abuser. 

This doesn’t just happen with parents to children though, and Juvia’s gaslighting in the grave scene is proof of that. She has guilt tripped and emotionally manipulated Gray into gaslighting himself on the severity of his own grief. He goes to comfort her because her manipulation and his own gaslighting has made him believe his grief is lesser, and her blushing and telling him he’s warm lets us know that she was never really guilty, she just wanted Gray to comfort and forgive her so that she had the go ahead to go right back to what she was doing to him before this. 

And by giving an affirmation like "you're warm", that could be read as her forgiving him, it's a positive affirmation that says that he's doing the correct thing by comforting her. So, in manipulating him into gaslighting himself, she doubles down on the manipulation by rewarding him for doing it. And, in a lot of cases of abuse, it's the moments of kindness and "love" that sew the seeds in a victim's head that what they're going through isn't actually abuse, because "they were nice to me this one time".

Therefore, it’s indirect gaslighting instead of direct gaslighting, both of which cause the person being gaslight to question something about their own reality, just in different ways, and contrary to popular belief, indirect gaslighting is more common than direct.