How to react to keypress events in Go?

Game engines commonly implement this kind of functionality. They are usually also pretty much platform agnostic (usually at least Windows, Linux, Mac OS X). Try for instance Azul3D's keyboard library.

The logic is off the top of my head something like

watcher := keyboard.NewWatcher()
// Query for the map containing information about all keys
status := watcher.States()
left := status[keyboard.ArrowLeft]
if left == keyboard.Down {
    // The arrow to left is being held down
    // Do something!
}

Getting a list of what keys are currently being pressed down is a matter of iterating the map through and listing the keys where value was Down.

This answer for a similar question points to few options depending on which platform you need to implement this in.

I've personally used https://github.com/MarinX/keylogger.
It's well written and easy to understand. At the time, I had to write my own version of this library to listen to multiple keyboards, It was easy to adapt this code for that.
Note that this library is written for Linux only.

Example code:

import (
    "github.com/MarinX/keylogger"
    "github.com/sirupsen/logrus"
)

func main() {

    // find keyboard device, does not require a root permission
    keyboard := keylogger.FindKeyboardDevice()

    logrus.Println("Found a keyboard at", keyboard)
    // init keylogger with keyboard
    k, err := keylogger.New(keyboard)
    if err != nil {
        logrus.Error(err)
        return
    }
    defer k.Close()

    events := k.Read()

    // range of events
    for e := range events {
        switch e.Type {
        // EvKey is used to describe state changes of keyboards, buttons, or other key-like devices.
        // check the input_event.go for more events
        case keylogger.EvKey:

            // if the state of key is pressed
            if e.KeyPress() {
                logrus.Println("[event] press key ", e.KeyString())
            }

            // if the state of key is released
            if e.KeyRelease() {
                logrus.Println("[event] release key ", e.KeyString())
            }

            break
        }
    }
}