Can I use the Watch with gps + cellular if I live in Mexico?

While the Watch is not locked to a carrier, and can use a variety of LTE bands, there are currently no carriers in Mexico that support the Watch. Cell carriers must support the watch in order to activate the internal digital sim card with their service.

Apple Watch - Carriers - Apple

Telcel is rumoured to be working on providing support in Mexico for the Series 3, but when this will be is still unknown.

Meanwhile, you can still use the Watch as a normal non cellular version but will be restricted for most uses to being around the paired iPhone.

Hi

Apple Watch Series 3 (GPS + Cellular) is not locked to any specific carrier and has a built-in eSIM that copies information from the iPhone's SIM card.

The built-in cellular features require a carrier plan in the country of use that specifically includes support for Apple Watch.

If buying Apple Watch Series 3 (GPS + Cellular) in another country, then you would need to buy a model that supports the relevant LTE and UMTS bands for your intended country of use and supported carrier.

There are three different regional versions of Apple Watch Series 3 (GPS + Cellular), each supporting different LTE and UMTS bands, as detailed here:

Apple Watch - Carriers - Apple

I suggest that you contact your preferred carrier in your intended country of use to ask whether any given model of Apple Watch Series 3 (GPS + Cellular) would be compatible with any service plan that they may offer or plan to offer for iPhone and Apple Watch Series 3 (GPS + Cellular).

Note that the built-in cellular features cannot be used when roaming. Also keep in mind that warranty service availability may be restricted.