Some of this does depend on the font, with which you have to play from there. I use EB Garamond (this test has the inferior original version, my documents specify which versions to use), and it unfortunately lacks a cross pattée (U+2720, one of the font's few failings that's hard to fix), so that would need to be corrected… but the dagger works for this test yours truly got the code wrong for Unicode and if you use the regular version, not the original, the problem goes away, but my question about Gregorio does still stand (see attached). Once the score is finished, you can adjust whether the symbol is under a bar line or on an empty syllable, which can be moved to a new line as necessary. Just this bit from the beginning might help.

BTW, what is the correct way to get a symbol (like +, but it doesn't have to be that) to appear a Maltese cross/cross pattée? (This would solve the issue, even if it's sort of ugly — the Passion and something like the blessing of the font are the only nexts where I need to do this in gabc, instead of calling a separate font in the .tex file itself, like for a blessing printed with no music.) The website gives \gredefsizedsymbol{GreCross}{greextra}{Cross}, but that printed the dagger for the psalms and flex of the prayers, and I really don't understand how this works with \grebarredsymbol.

this is my .tex file (just in case that helps answer my question and help solve the first one too; it won't let me attach it):

\documentclass[11pt]{article}
\usepackage{fontspec}
\setmainfont{EB Garamond}
\usepackage[autocompile]{gregoriotex}
\gredefsizedsymbol{GreCross}{greextra}{Cross}

\begin{document}
\gregorioscore{Passion-test}
\enddocument

Thank you immensely.
My initial reaction was: "Such knowledge is too wonderful for me. I cannot attain to it" but I'm going to try to follow your suggestions.

I used the Meinrad fonts previously, and I found them much more difficult to use than gabc. Furthermore, Windows 11 does not respond when I try to invoke symbols accessible through alt + number.