The gadsden purchase is a tiny fraction of what we took from Mexico. It is basically the southern 25% of AZ (at most) and a small piece of NM. See
Gadsden Purchase - Wikipedia, the free encyclopedia
The entire states of California, Nevada, Utah, most of Colorado, most of Arizona and almost all of New Mexico were taken from the Mexicans in the war that started in 1845. Texas we got by annexing it 10 years after Anglo settlers declared independence from Mexico in 1835. We didnt pay them for any of this.