When Compounds are Considered Compounds and Their Types
A molecule is formed when two or more atoms join together chemically e.g. Na and Cl.

A compound is a molecule that contains at least two different elements e.g. Na and Cl.

All compounds are molecules but not all molecules are compounds e.g. H2 and O2 are molecules but are not compounds because bond is between two of the same elements.

Im not sure what you are asking in question 3. Gay-Lussacs Law is generally associated with the combining volumes principle. e.g. 1 volume of H2(g) + 1 volume of O2(g) = 2 volumes of H2O(g). However, you may want to consider the combined gas law, not just Gay-Lussac law. This combines Gat-Lussacs law with Boyles' and Charles' Law.