Keras objects cannot be saved like other R objects.
  The methods as.raw() and as.keras() can
  be used to convert the model slots of objects
  of S3 class "gnn_GNN" to
  "raw" objects (which can be saved)
  or "keras.engine.training.Model" objects (which
  can be trained).
# S3 method for gnn_GNN
as.raw(x)
# S3 method for gnn_GNN
as.keras(x)object of S3 class "gnn_GNN" with
  slot method converted by the respective method if necessary.
object of S3 class "gnn_GNN".
Marius Hofert