from fastapi import APIRouter, HTTPException

users = []  # In-memory storage

router = APIRouter()

@router.get("/auth")
async def authenticate_user():
    """authenticates the user"""
    return {
        "method": "GET",
        "_verb": "get"
    }

```

This code defines a GET endpoint at `/auth` using FastAPI's `APIRouter`. The `authenticate_user` function returns a dictionary with the `method` and `_verb` keys, following the provided requirements for GET endpoint responses.

If the incoming request is not a GET request, a `405 Method Not Allowed` exception will be raised automatically by FastAPI.