The differential of $\Phi$ at a point $(D,\theta)$ in the fiber product is given by the induced map on tangent spaces:

$$T(\mathrm{Div} _{\ge 0}(X)) \times T(\mathrm{Pic} (X)) \times T(\mathrm{Aut} ^0 (X)) → T(\mathbb G (n,|\mathcal L|))$$

where $T$ denotes the tangent space.

To compute this differential, we can use the fact that the tangent space of $\mathrm{Div} _{\ge 0}(X)$ at a point $D$ is isomorphic to $H^0(X,\mathcal O _D)$, the space of global sections of the structure sheaf of $X$ twisted by $D$. Similarly, the tangent space of $\mathrm {Pic} (X)$ at a point $\mathcal L$ is isomorphic to $H^1 (X, \mathcal O _{\mathcal L})$, the space of global sections of the sheaf of holomorphic sections of $\mathcal L$. Finally, the tangent space of $\mathrm{Aut} ^0 (X)$ at the identity is isomorphic to $H^0 (X,TX)$, the space of vector fields on $X$.

Using these identifications, we can write down the differential of $\alpha$ and $\beta$, and the map induced by $Q \subseteq |\mathcal M|$ on the space of global sections of $\mathcal L$. The differential of $\alpha$ is given by the map

$$T(\mathrm{Div} _{\ge 0}(X)) \ni f \mapsto \mathcal O _X (f+D) \otimes \mathcal O _X (D)^{-1} \in T(\mathrm{Pic} (X))$$

while the differential of $\beta$ is given by the map

$$T(\mathrm {Aut} ^0 (X)) \ni V \mapsto \mathcal L \otimes V^* \mathcal M ^{-1} \otimes \mathcal M \otimes {\mathcal L} ^{-1} \in T(\mathrm{Pic} (X)) \ . $$

To compute the differential of the map induced by $Q$, we need to consider the linear system $Q$ as a subspace of $H^0 (X, \mathcal M)$. Let $s_1, \dots, s_n$ be a basis for $Q$, and let $t_1, \dots, t_m$ be a basis for $H^0 (X, \mathcal M)$ such that $s_1, \dots, s_n, t_1, \dots, t_m$ is a basis for $H^0 (X, \mathcal M)$. Then the map induced by $Q$ is given by the matrix

$$[ s_1, \dots, s_n, t_1, \dots, t_m ] \ .$$

To compute the differential of $\Phi$, we use the product rule for differentials:

$$\mathrm d \Phi (D, \theta) = \mathrm d (D + \theta^* Q) + (\mathrm d \alpha(D), \mathrm d \beta(\theta)) + (\mathrm d (Q),0) \ ,$$

where $\mathrm d (Q)$ is the differential of the map induced by $Q$. The first term on the right-hand side is just the differential of the translation map $T_{\mathrm {Div} _{\ge 0} (X)}$ given by $\mathrm d (D + \theta \wedge Q) = \mathrm d D + \theta \wedge \mathrm d Q$, while the second term is the product of the differentials of $\alpha$ and $\beta$. The last term is zero, since the differential of $Q$ does not depend on $\theta$.

Putting everything together, we obtain the differential of $\Phi$ at a point $(D, \theta)$ in the fiber product.