code: def test_gls(self):
      p = self.p + stat_smooth(
          method="gls", formula="y ~ np.sin(x)", fill="red", se=True
      )
      assert p == "gls_formula"
  
imports: import numpy
  import pandas
  import pytest
  import statsmodels.api
  from plotnine import aes, coord_trans, geom_point, geom_smooth, ggplot, stat_smooth
  from plotnine.exceptions import PlotnineWarning
label_desc: Use image node_id 5 for calling the TestFormula obj's underlying member method code with example usage: obj.test_gls() without return types