if you purchase the Apple Watch series 3 gps only can you text and talk?

Hi

Yes - Apple Watch Series 3 (GPS) can be used in conjunction with a compatible iPhone and a service plan for the iPhone from any carrier for calls and texts, subject to connectivity:

• Send messages
• Make phone calls

Apple Watch Series 3 (GPS) requires an iPhone 5s or newer model with the latest version of iOS 11.x:

Set up your Apple Watch - Apple Support

Functions that are available on Apple Watch when it is away from your iPhone depend on various factors including:

• Your Apple Watch model:

  Apple Watch - Compare - Apple

• The features of your carrier service plan:

  Make a call with Wi-Fi Calling - Apple Support

• The type of connectivity that is available to your Apple Watch and/or iPhone at the time:

  About Bluetooth, Wi-Fi, and cellular on your Apple Watch - Apple Support

Features that are available in different cases are detailed here, including within the footnotes:

How to use your Apple Watch without your iPhone nearby - Apple Support