I do not really answer you question but maybe this helps:

Let $\mathcal{K} =\mathbb{C}((t))$ and $\mathcal{O}:=\mathbb{C}[[t]]$. For $n\geq 0$ denote the $\mathcal{K}_n$ the $\mathcal{O}$ ideal in $\mathcal{K}$ generated by $t^{-n}$. Choose an embedding $G\hookrightarrow GL_m$. Let
$$G(\mathcal{K}_n):=\{A\in G(\mathcal{K})\vert \text{ all entries of } A \text{ and } A^{-1}  \text{ are in }\mathcal{K}_n\}$$

Then $$Gr_n=G(\mathcal{K}_n)/G(\mathcal{O})$$

Via this you can in principle calculate all $\lambda$ which are in $Gr_n$. (Note that you can write every $\lambda$ in the form $\lambda: \mathbb{C}^\times \to T$, $t\mapsto \begin{pmatrix}t^{\lambda_1} & &  \\  & \ddots \\ && t^{\lambda_m} \end{pmatrix}$ for some $\lambda_i\in \mathbb{Z}$ and all unspecified entries are zero.)