antenna macbook

Just a guess, but I don't think it has significant gain per se. Tiny convoluted wires of likely 1/4 wavelength (or [smaller] multiples thereof) generally don't exhibit significant gain. Remember, it's inside a reflective metal case with some non reflective, or less reflective, plastic or glass areas ... the screen or the keyboard depending on where the little wire is.

If the concern is the positioning of the Mac changes its ability to use or stay connected to a router, that's not something you can change on the Mac. You may can enhance a router's signal or focus it with newer models.

Whatever the gain is, it is directional in nature. Given the positioning of the antenna inside the case, and the orientation (polarization: horizontal, vertical, circular or mixed) of the router's signal, gain can certainly be directional. In fact, it has to be.

If that's not the concern, what is?

Assume unity for your purposes.

Do the research on antenna gain and see for yourself. If you don't understand "unity" you don't understand gain.

Additionally, Apple is not going to publish their trade secrets.

https://www.everythingrf.com/community/how-do-you-measure-antenna-gain