Olá Murilo!

Sua pergunta está bem resumida mas acredito que o que você quer é adicionar campos na sessão do usuário logado.

Eu faço assim:

1- Criar uma classe descendente da GrailsUser:

public class WebUser extends GrailsUser {
        
        private static final long serialVersionUID = -6212547755684175363L;
        private MapParameter parameter = new MapParameter();
        private boolean superUser;

        public WebUser(String username, String password, boolean enabled,
                        boolean accountNonExpired, boolean credentialsNonExpired,
                        boolean accountNonLocked, boolean superUser, 
                        Collection<GrantedAuthority> authorities, Object id) {
                super(username, password, enabled, accountNonExpired, credentialsNonExpired,
                                accountNonLocked, authorities, id);
                this.superUser = superUser;
        }        
        
        public boolean isSuperUser() {
                return superUser;
        }        
        
        public void setParams(Map<String, ?> params) {
                parameter.clear();
                for (String key : params.keySet())
                        parameter.setObject(key, params.get(key));                
        }
        
        public Parameter getParams() {
                return parameter;
        }

}

2- Criar serviçõ que implementa a interface GrailsUserDetailsService

class UsuarioService implements GrailsUserDetailsService {
          
        UserDetails loadUserByUsername(String username)        throws UsernameNotFoundException, DataAccessException {
                return loadUserByUsername(username, true)
        }

        UserDetails loadUserByUsername(String username, boolean loadRoles) throws UsernameNotFoundException, DataAccessException {                
                Usuario.withTransaction { status ->                        
                        def usuario = Usuario.findByLogin(username) 
                        if (!usuario) throw new UsernameNotFoundException( 'User not found', username)
                        
                        def acessos = (loadRoles) ? usuario.acessos : []
                        acessos << new GrantedAuthorityImpl(AuthenticatedVoter.IS_AUTHENTICATED_FULLY)
                        
                        def user = new WebUser(usuario.login, usuario.senha, usuario.ativo, !usuario.expirado, 
                                !usuario.senhaExpirada, !usuario.bloqueado, usuario.administrador, acessos, usuario.id)
                                                                                                                
                        return user
                }
        }
        
}

3- Adicionar referencia em resources.groovy

beans = {
  userDetailsService(UsuarioService)
}