In the cloud_Pub_Sub.js file paste below code

Create cloud_Pub_Sub.js file

import { LightningElement, track } from 'lwc';
import pullMessages from '@salesforce/apex/GoogleCloudPubSubService.pullMessages';
export default class Cloud_Pub_Sub extends LightningElement {
    @track messages = [];
 
    getMessages() {
        console.log('onclick');
        try {
            pullMessages()
            .then(result => {
                this.messages = result;
                console.log('got message', this.messages);
            })
            .catch(error => {
                console.error('Error in pullMessages: ', error);
            });

        } catch (e) {
            console.error('Error in getMessages: ', e);
        }
    }

}