https://golang.org/pkg/builtin/#int

int is a signed integer type that is at least 32 bits in size. It is a distinct type, however, and not an alias for, say, int32.

The textbook you are looking for is

The Go Programming Language Specification

Numeric types

A numeric type represents sets of integer or floating-point values. The predeclared architecture-independent numeric types are:

uint32 set of all unsigned 32-bit integers (0 to 4294967295)
uint64 set of all unsigned 64-bit integers (0 to 18446744073709551615)

int32  set of all signed 32-bit integers (-2147483648 to 2147483647)
int64  set of all signed 64-bit integers (-9223372036854775808 to 9223372036854775807)

There is also a set of predeclared numeric types with implementation-specific sizes:

uint either 32 or 64 bits
int  same size as uint

Check the size of type int. On the Go Playground it's 4 bytes or 32 bits.

package main

import (
    "fmt"
    "runtime"
    "unsafe"
)

func main() {
    fmt.Println("arch", runtime.GOARCH)
    fmt.Println("int", unsafe.Sizeof(int(0)))
}

Playground: https://play.golang.org/p/2A6ODvhb1Dx

Output (Playground):

arch amd64p32
int 4

Run the program in your (LeetCode) environment. It's likely 8 bytes or 64 bits.

For example, in my environment,

Output (Local):

arch amd64
int 8

Here are some fixes to your code,

package main

import (
    "fmt"
    "runtime"
)

func main() {
    fmt.Println(runtime.GOARCH)
    fmt.Printf("%v\n", singleNumber([]int{-2, -2, 1, 1, -3, 1, -3, -3, -4, -2}))
}

func singleNumber(nums []int) int {
    sum := make([]int, 64)

    for _, v := range nums {
        for i := range sum {
            sum[i] += 1 & (v >> uint(i))
        }
    }

    res := 0
    for k, v := range sum {
        if (v % 3) != 0 {
            res |= (v % 3) << uint(k)
        }
    }
    fmt.Printf("res %+v\n", res)
    return res
}

Playground: https://play.golang.org/p/kaoSuesu2Oj

Output (Playground):

amd64p32
res -4
-4

Output (Local):

amd64
res -4
-4