When not using parts of the house, is it recommended to close the vents in those rooms or does this effect the efficiency of the gas pack system? Thanks

It usually is not recommended to close off regiters because that messes with air circulation. I am not familiar with the term gas pack but in air conditioning it is a no-no.

It can change the static pressure that the blower has to push against. Some units can handle it, some cannot.

We empty nesters have kept the vents in 3 of our 4 bedrooms closed for years, summer and winter.