Gas.... well, primarily, a short-term concern for the pilot(s), right ? Relatively easy to address, and necessary close to a volcano.
Otherwise, I am shure they are not addressed at all. The ´ASH advisory centers´ deal with ash... almost no-one except athmospheric scientists takes the non-ash stuff seriously.
Look at the Raikoke-eruption. Ash cleared after a couple of days, huge gas cloud stayed over eastern Siberia.
Didn´t seem to bother flights...in the short term.
?