As soon as you have one employee you will have a personal file about them and you will process the data of your employee as part of the personnel accounting. Due to this processing of data also your employees come under the GDPR.
What you should also keep in mind is that you are not only going to process normal personal data of your employees but also so-called special categories of personal data, like e.g. the religious affiliation of your staff to give your employees days off because of holidays or the trade union membership of your employees, cf. Article 9.
Besides, these special categories of personal data merit specific protection as the context of their processing could create significant risks to the fundamental rights and freedoms, e.g. the publication of the religious affiliation could lead to disadvantages or bullying of one employee.