Defining custom Gnome compose sequences

Question

I understand this question has been asked before (e.g. here), but some time has passed and some things may have changed. Also, my problem is that the suggested solutions aren't working.

I want to define a custom compose sequence in GNOME. I am running Ubuntu 19.04. The sequences I want to define are:

<Multi_key> <bar> <A>           : "A̍"  # LATIN CAPITAL LETTER A WITH VERTICAL LINE ABOVE
<Multi_key> <bar> <a>           : "a̍"  # LATIN SMALL LETTER A WITH VERTICAL LINE ABOVE
<Multi_key> <bar> <E>           : "E̍"  # LATIN CAPITAL LETTER A WITH VERTICAL LINE ABOVE
<Multi_key> <bar> <e>           : "e̍"  # LATIN SMALL LETTER A WITH VERTICAL LINE ABOVE
<Multi_key> <bar> <I>           : "I̍"  # LATIN CAPITAL LETTER A WITH VERTICAL LINE ABOVE
<Multi_key> <bar> <i>           : "i̍"  # LATIN SMALL LETTER A WITH VERTICAL LINE ABOVE
<Multi_key> <bar> <O>           : "O̍"  # LATIN CAPITAL LETTER A WITH VERTICAL LINE ABOVE
<Multi_key> <bar> <o>           : "o̍"  # LATIN SMALL LETTER A WITH VERTICAL LINE ABOVE
<Multi_key> <bar> <U>           : "U̍"  # LATIN CAPITAL LETTER A WITH VERTICAL LINE ABOVE
<Multi_key> <bar> <u>           : "u̍"  # LATIN SMALL LETTER A WITH VERTICAL LINE ABOVE


<Multi_key> <grave> <M>         : "M̀"  # LATIN CAPITAL LETTER M WITH GRAVE
<Multi_key> <grave> <m>         : "m̀"  # LATIN SMALL LETTER M WITH GRAVE
<Multi_key> <asciicircum> <M>       : "M̂"  # LATIN CAPITAL LETTER M WITH CIRCUMFLEX
<Multi_key> <asciicircum> <m>       : "m̂"  # LATIN SMALL LETTER M WITH CIRCUMFLEX
<Multi_key> <underscore> <M>        : "M̄"  # LATIN CAPITAL LETTER M WITH MACRON
<Multi_key> <underscore> <m>        : "m̄"  # LATIN SMALL LETTER M WITH MACRON



<Multi_key> <apostrophe> <N>        : "Ń"   # LATIN CAPITAL LETTER N WITH ACUTE
<Multi_key> <apostrophe> <n>        : "ń"   # LATIN SMALL LETTER N WITH ACUTE
<Multi_key> <asciicircum> <N>       : "N̂"  # LATIN CAPITAL LETTER N WITH CIRCUMFLEX
<Multi_key> <asciicircum> <n>       : "n̂"  # LATIN SMALL LETTER N WITH CIRCUMFLEX
<Multi_key> <underscore> <N>        : "N̄"  # LATIN CAPITAL LETTER N WITH MACRON
<Multi_key> <underscore> <n>        : "n̄"  # LATIN SMALL LETTER N WITH MACRON

I looked at the comments in the GNOME source file gtk+-2.10.7/gtk/gtkimcontextsimple.c as described here and it wants me to create a file ~/.config/gtk-4.0/Compose which I did and added those lines (encoded in UTF-8). I've restarted my computer but it simply does not work. I'm at at loss at why and how to debug this, so looking for some help!