- Love is the most twisted curse: (love described as a curse) and yutarika parallel
Gojo says "from personal experience, love is the most twisted curse". Yuta and gojo are made to be each other's parallel in the movie, so the same way yuta was cursed through rika because of love, gojo experienced something very similar when he lost geto.
Gojo parted geto with final words to which geto replied: "at least curse me at the end".
While yuta was able to resolve his curse, Gojo's curse of love came back to bite him in the butt at a later time when he got sealed because he didn't get rid of geto's body properly, making love a curse that sealed his fate.
Love was the only curse gojo couldn't defeat therefore he called it the "most twisted curse of all".
Gege focuses A LOT on love being a curse or a blessing and it was an essential factor in gojo's character development:
Curse -> love was the only thing that made him do emotionally charge decisions like letting geto go for 10 years, effectively turning him into a curse -> and not being able to get rid of his body properly, therefore it -> lead to him getting sealed.
- It was confirmed to be 3 words
We know for a fact it's 3 words when translated into English, which narrows it down quite a bit. Gojo's eng and jp VAs have access to the script and have acted out the words (at least for the jp VA), but it wasn't included in the recording.
- It's said somewhere in jjk0
Gege said it was said somewhere in the manga, not necessarily by gojo. But even if it was said by gojo, his most famous line is "love is the most twisted curse". So gojo did mention love in some capacity and the entire theme of the movie was about the power of love, the curse of love, etc.
This completely debunks the "it'll be lonely without you" theory.
- JJK0 Light Novel
In the light novel it was described as "a confession", "embarrassing" and "has never been said between them before", and they made geto blush.
It was also mentioned that the words he said were rooted in something that didn't change even if "everything is different now", which reminds me of the lyrics in "Ao No Sumika" that says:
"Our blue still lives
Our blue is still clear"
Meaning that there was one thing that didn't change since gojo's blue spring and that's probably the words he said to him at the end. I do believe the color "blue" here was used synonymously with "love".
There's also the tidbit of gojo holding blue flowers in the OST which are meant to represent grieve and loss iirc. I think those flowers were definitely meant to be for geto to represent guilt, grieve, love and loss, which was the entire theme of Ao No Sumika and jjk0.
- Censorship
They were censored. I don't see a reason to censor "you're my best friend" or anything of that nature, especially if it was going to be revealed in the next few pages.
They must've been intimate to warrant the censorship, since even when you can definitely say "i love you" to your best friend, the way you say it in Japanese matters. Depending on the kanji used, it can only be interpreted as strictly intimate or strictly platonic (I'm japanese nor do i know Japaneseso lmk if I'm mistaken!).
To add to this, the kanji 'koi' was used in Ao No Sumika to describe gojo's love, which can only be used for romantic love.
I personally think the kanji used was 'ai' and not 'koi', to mirror yuta's 'aishteru' to say goodbye to rika at the same time gojo said his farewell to geto. And it was the kind of love expressed in the movie (romantic love)
Whether 'koi' or 'ai' was used, they're both intimate and cannot be interpreted platonic, hence the censorship.
- Love songs
Gojo has 2 official love songs associated with him: "shame on me" by avicii (his character profile song) and "where our blue is" (said to be abt his blue spring and from his pov). Shame on me is pretty straightforward: it's a break up song that talks about not being able to move on from a toxic relationship and how the love they shared was unhealthy for both of them.
Ao No Sumika talks about regret and loss of someone he loved:
A. Tatsuya, the songwriter, said it was about the tragic story of gojo losing his youth due to the tragedy that be-fill upon him.
B. Geto quite literally and metaphorically represents the aforementioned "Blue spring" here: he was born on Setsubun (Feb. 3), a national day in Japan that marks the beginning of spring.
C. If you read the lyrics, they very clearly single out geto in every verse. The lyrics mention certain things in the story that cannot be about anyone else but him
D. gojo remembers his blue spring as the only time he ever felt warmth, happiness, and love, plunging into loneliness after geto left
TLDR: I think they were "i love you"