The input to the gain block is
\[X=V_{in}-V_{out}\]
the output of the gain block is
\[V_{out}=H(s) X=H(s) (V_{in} - V_{out}) = H(s) V_{in} - H(s) V_{out}\]
rewriting this with \[V_{out}\] on the left and \[V_{in}\] on the right we get
\[(1 + H(s)) V_{out}= H(s) V_{in}\]
rearranging by cross multiplying gives
\[ \frac{V_{out}}{V_{in}} = \frac{H(s)}{1+H(s)}\]