If you're just needing a simple arrow, I'd just draw the whole thing including the arrowhead:
%start
simpleArrow =
#'((lineto 3 0)
(lineto 2 0.75)
(lineto 3 0)
(lineto 2 -0.75))
\markup {
\path #0.25 #simpleArrow
}
%end
Or if you need something fancier, maybe you can find something in the LSR:
https://lsr.di.unimi.it/LSR/Search?q=arrow
I didn't really answer your question about the gap though, sorry about that
