Aqua regia is used for dissolving noble metals (Au, Pt, etc.). The gas evolved in this process is : 

$\begin{array}{l}\mathrm{Au}+4{\mathrm{H}}^{(+)}+{\mathrm{NO}}_3^{(-)}+4{\mathrm{Cl}}^{(-)}\to {\left[{\mathrm{AuCl}}_4\right]}_2^{(-)}+\mathrm{NO}+2{\mathrm{H}}_2\mathrm{O}\\ 3\mathrm{Pt}+16{\mathrm{H}}^{(+)}+4{\mathrm{NO}}_3^{(-)}+18{\mathrm{Cl}}^{(-1)}\to 3{\left[{\mathrm{PtCl}}_6\right]}_2^{(-)}+4\mathrm{NO}+8{\mathrm{H}}_2\mathrm{O}\end{array}$

So Aquaregia is used to dissolve Au, Pt 

Here, NO gas releases.