f(x) = sum([x*a>1 for a in gaussian(...)]) % some normal distribution

pgfmathdeclarefunction{gaussian}{2}{pgfmathparse{1/(#2*sqrt(2*pi))*exp(-((x-#1)^2)/(2*#2^2))}%
}

pgfmathdeclarefunction{gsum}{2}{%
pgfmathsetmacroret{0}%
foreach i in {#1}{%
  pgfmathsetmacroret{ret+(i*#2>1?1:0)}%
  xdefret{ret}%
}%
pgfmathparse{ret}%
}

...

addplot [
domain=0:1.5,
samples=100,
color=red,
]
{gsum(gaussian(0.75,0.25), x)};