I believe Gain is intentionally distinguished from Pick Up and Trade and Preston's card specifically says Gain.
I would agree as the Trade, Pick Up and Steal Actions do not mention "Gain" but it is defined separately and used in the App
^^When an investigator gains a Common Item, Unique Item, or
Condition, he claims that card, placing it faceup in his play area.
^^When an investigator gains a Spell, he claims a random copy of
that card, placing it faceup (art side up) in his play area.
^^When an investigator gains a Clue, he claims that token, placing
it in his play area.
We played him earlier able to Trade (drop and then pickup) items removing/flipping a Horror as he went once per round. Although this could be viewed as him admiring his acquisitions, or when the Trade Action is initiated by another player that he is feeling pretty pumped about something new. Which kind of works as he can't use it to benefit another player anyway, but someone potentially loses an action for it.