What’s the difference between Gay lusacs and Avogadro’s Law?

Gay lussac's law of combining volumes says that whenever gases combine , they do so in volumes which bear a simple whole number ratio to each other.

Avogadro's law says that under similar temperature and pressure conditions  equal volumes of gases contain equal number of "molecules"  or contain equal moles of the gases. For eg , 22.4L of any gas will contain one mole of molecules of the gas at NTP.

We know that 1 mole of every substance has 6.022x10^23 particles.

Same is the Avogadro law. It says that equal amounts of gases at same physical conditions (same temperature, pressure) have same number of particles. The fact that 1 mole of every substance has 6.022x10^23 particles is an extension of this law only. That is why the number 6.022x10^23 is called Avogadro’s number. 

 

In Gay Lussac’s law of Gaseous Volume, it is said that when two gases combine, they combine in ratios of whole numbers.

This means gases cannot combine in ratios like 0.143:3.55 or 1.66:32, etc. It is always a whole number ratio like 1:2, 3:1 etc