package models;

import javax.persistence.*;

@Entity
@Table(name = "group_types")
public class GroupType {
    @Id
    @GeneratedValue
    @Column(name = "group_type_id")
    private long id;

    @Column
    private String type;

    public long getId() {
        return id;
    }

    public String getType() {
        return type;
    }

    public void setType(String type) {
        this.type = type;
    }
}

package models;

import modules.preloader.DatabasePreloader;

import javax.persistence.*;

@Entity
@Table(name = "group_types")
public class GroupType {
    @Id
    @Column(name = "group_type_id")
    private String id;

    @Column
    private String type;

    public String getId() {
        return id;
    }

    public String getType() {
        return type;
    }

    public void setType(String type) {
        this.type = type;
    }

    public GroupType() {
    }

    public GroupType(String id, String type) {
        this.id = id;
        this.type = type;
    }

    static {
        DatabasePreloader.addDefault((em -> {
            em.persist(new GroupType("selfGroup", "Person Group"));
            em.persist(new GroupType("conversation", "Conversation"));
            em.persist(new GroupType("social", "Social group"));
        }), 0);
    }
}

Change GroupType.id to String, add default values