I'm reading Van Lint's Course in Combinatorics:
He mentions $\Gamma(a)$ in this text but I'm not really sure of what it means and I'm also afraid of assume something wrong, at first thought I assumed that it would be some vertice in $G$ and hence $\Gamma(A)$ would be all the vertices of a subset. I've skimmed the previous chapters of the book and found no mention to $\Gamma$.
$\Gamma(a)$ is the set of vertices adjacent to $a$. In the bipartite graph $G$ whose vertices are bipartitioned into the sets $X$ and $Y$, $\Gamma(x)\subseteq Y$ for each $x\in X$, and $\Gamma(y)\subseteq X$ for each $y\in Y$.