Hi Artur
Sven has cleared up the reason for my query, relating to the selection of Custom Model option which over-rides the default model types (LL LLT BSM).
I am using the 2021a version of Gretl with StrucTism 24 /12/ 20.
Nevertheless, in a model specified with stochastic irregular, stochastic trend and stochastic slope options, I would have anticipated the forecasts would not be perfectly linear. Perhaps this reflects my lack of awareness of the form of the forecast function in the module.
Sorry I can't provide script as I am using the GUI.
Brian