An example of how to find difference between two slices in Go. It uses map and empty struct (0 bytes) as a void to convert slice to map (kind of set).

twoSlicesDiffs.go

/*
An example of how to find the difference between two slices.
This example uses empty struct (0 bytes) for map values.
*/

package main

import (
	"fmt"
)

// empty struct (0 bytes)
type void struct{}

// missing compares two slices and returns slice of differences
func missing(a, b []string) []string {
	// create map with length of the 'a' slice
	ma := make(map[string]void, len(a))
	diffs := []string{}
	// Convert first slice to map with empty struct (0 bytes)
	for _, ka := range a {
		ma[ka] = void{}
	}
	// find missing values in a
	for _, kb := range b {
		if _, ok := ma[kb]; !ok {
			diffs = append(diffs, kb)
		}
	}
	return diffs
}

func main() {
	a := []string{"a", "b", "c", "d", "e", "f", "g"}
	b := []string{"a", "c", "d", "e", "f", "g", "b"}
	c := []string{"a", "b", "x", "y", "z"}
	fmt.Println("a and b diffs", missing(a, b))
	fmt.Println("a and c diffs", missing(a, c))
}

/*
Output:

a and b diffs []
a and c diffs [x y z]
*/

Nice approach with maps.
Unfortunately, this won't work if slices have repeating elements.
a := []string{"a", "b", "b"}
b := []string{"a", "a", "b"}
Still results in output a and b diffs []

And having extra element in the first slice that is not present in the second slice goes unnoticed as well.
a := []string{"a", "b", "c"}
b := []string{"a", "b"}
Also results in output a and b diffs []