In Section 13.2 of *Applied longitudinal analysis*, it says:

> For the case of linear models (i.e., marginal models with an identity link function), the generalized least squares (GLS) estimator of $\beta$ discussed in Chapter 4 can be considered a special case of the GEE approach.


## Reference
Fitzmaurice, Garrett M., Nan M. Laird, and James H. Ware. Applied longitudinal analysis. Vol. 998. John Wiley & Sons, 2012.