Let $X$ be a regular scheme which is flat over $\mathbf{Z}$. The arithmetic Grothendieck group $\hat{K}(X)$ is defined to be the quotient of $\hat{G}(X)$ by $\hat{G}^\prime(X)$. This is actually quite a length definition which I added below for the sake of completeness.

In the classical case, for $X$ any noetherian scheme, the Grothendieck group $K_0(X)$ is defined to be the Grothendieck group of the category of vector bundles on $X$. That is, one applies the notion of a Grothendieck group for an additive subcategory of an abelian category. (In our case the abelian category is the category of coherent sheaves on $X$.) This means just modding out by short exact sequences.

I would like to know if there is a categorical type of definition for this group. Thus, first one needs to decide what kind of categories we're talking about (objects are pairs in some sense) and then the notion of exact sequence should coincide in some sense with the below definition.

Probably there is no such thing. I just ask this question in order to understand the arithmetic Grothendieck group better.

Note. Let me sketch the definition of the arithmetic Grothendieck group as given in Faltings. In the above $\hat{G}(X)$ is the direct sum of "the free abelian group generated by all vector bundles which have a hermitian metric on $X_{\mathbf{C}}$ which is invariant under complex conjugation $F$" and the abelian group $\widetilde{A}^\ast(X)$. Here $\widetilde{A}^\ast(X)$ is generated by all $p$-forms $\alpha^p$ such that $F^\ast \alpha^p = (-1)^p \alpha^p$. Furthermore, $\hat{G}^\prime(X)$ is the subgroup generated by elements of the form $E_2 - E_1-E_3 - \widetilde{ch}(E)$, where $E$ is the short exact sequence $$0\rightarrow E_1 \rightarrow E_2 \rightarrow E_3 \rightarrow 0$$ and $\widetilde{ch}(E)$ is the secondary Chern form.