Lanolin is used in experiments to deliver growth hormones to plant stems. It was first used by Laibach in the 1940s as a paste with plant extracts. Lanolin is still used because it has the ability to be applied to plant stems to deliver the hormones without altering the plant or the dissolved molecules. It is fairly inert so does not influence the plant making it a good delivery vehicle while being able to form a semipermeable protective film of any cut surfaces on the plant. The lanolin restores the epidermal barrier function.
The lanolin will provide a similar barrier to environmental influence that the gibberellin will provide, hence you can attribute any effects of gibberellin to the hormonal influence, not the mechanical influence which will also be shown by the lanolin on the control group.