If you are planning to measure CH4 generated from horse dung, the implication is that you can do headspace sampling (with a gas-tight syringe).
Similar sampling could be applied to, e.g.,
1) rubbing alcohol (isopropanol),
2) nail varnish & nail varnish remover (acetone/ethyl acetate ??),
3) after-shave or perfume (ethanol)
4) (non-latex) paints & varnishes (toluene/xylenes)
5) paint stripper (dichloromethane/ toluene/ acetone ??)
Please run these suggestions by your teacher BEFORE proceeding.
Let me applaud your high school for having a functional gas chromatograph & a sharp science teacher.
I would encourage you to pursue a career in science, especially analytical chemistry!!!!!
It has certainly given me a wonderful life.
Please let us know how any of these ideas work out (or not).
Regards,
Motoball