  We give a new proof for the existence of rotationally symmetric steady and
expanding gradient Ricci solitons in dimension $n+1$, $2\le n\le 4$, with
metric $g=\frac{da^2}{h(a^2)}+a^2d\,\sigma$ for some function $h$ where
$d\sigma$ is the standard metric on the unit sphere $S^n$ in $\mathbb{R}^n$.
More precisely for any $\lambda\ge 0$, $2\le n\le 4$ and $\mu_1\in\mathbb{R}$,
we prove the existence of unique solution $h\in C^2((0,\infty))\cap
C^1([0,\infty))$ for the equation
$2r^2h(r)h_{rr}(r)=(n-1)h(r)(h(r)-1)+rh_r(r)(rh_r(r)-\lambda r-(n-1))$,
$h(r)>0$, in $(0,\infty)$ satisfying $h(0)=1$, $h_r(0)=\mu_1$. We also prove
the existence of unique analytic solution of the about equation on $[0,\infty)$
for any $\lambda\ge 0$, $n\ge 2$ and $\mu_1\in\mathbb{R}$. Moreover we will
prove the asymptotic behaviour of the solution $h$ for any $n\ge 2$,
$\lambda\ge 0$ and $\mu_1\in\mathbb{R}\setminus\{0\}$.
