Добавлю свои 5коп...
<script>
const str = `===============
f fgyuf
с Scryptics
s Nartelis19
rhvrihvieg445gv54vuv
===============
gk qwerty gfhfg
с h5vvh5vh5vh5
s bj65v54gc4g
gdfgdsggrsgdg433
===============
dgf ffrffef
c c4ccf4cf4cf4fc4
s Ugniukas1
v4gv4v5g4g4vg5v4
===============
gk qwerty kfefdf
c vg545v4gv45gv5g4v5g4
s jb6jb6jb6j
g4g4vv4gv4gvg4
===============
gtth vdvfv gtggg
c vg4vg45v45gvg4545vgv45g
s v454vgv545v4gvg45
v4gg4vvg4g4vvg4vg45vg45vg4545vg
===============
fvdver 443342
c cffx2sx2xs2xs2
s xe332xd23f4v5gh5b6
445vgv45gv45gv45gv45g45vg
===============
`;
const txt = 'gk qwerty'
let i = 0
do {
i = str.indexOf(txt, i)
if (i !== -1) {
i = str.indexOf('\n', i)
let k = i
for (let j = 0; j < 2 ; j++) k = str.indexOf('\n', k + 1)
const val = str.substring(i + 1, k).replace('\n', ': ')
document.write('<p>' + val + '</p>')
i = k
}
} while (i !== -1)
</script>