Gandalf is one of the many characters confirmed for Sméagol/Gollum's personality before Lord of the Rings. However, this timeline placement means many characters from the books (and films) can appear, and perhaps Gandalf will play the largest role in its narrative - even if he's mostly behind the scenes.
Based on the timeline of Lord of the Rings: Gollum, which traces Gollum's escape from Mordor and captivity by the Mirkwood Elves, it's likely Gandalf will be serving as more of an antagonist in the game (to Gollum, at least). During this time, as outlined in the books and in the Appendices in Return of the King, Gandalf began to suspect Bilbo had the One Ring and he started hunting after Gollum for more answers. However, at this time, Gollum wandered into Mordor and was captured and tortured by Sauron's minions.
Peter Jackson's Lord of the Rings movies skipped some of the finer details here but did an overall good job of tracing Gollum's whereabouts during Bilbo's - and eventually Frodo's - time with the One Ring. However, the movies made these events look like they took place over days or weeks, rather than years. Since Lord of the Rings: Gollum is inspired by the books, it stands to reason that the game will follow a more accurate timeline to Tolkien's work.
Lord of the Rings: Gollum Timeline Hints At Gandalf's Motives
According to Appendix B in Lord of the Rings, Gandalf begins looking for Gollum in 3001 with Aragorn's help, right after Bilbo leaves the Shire and leaves Frodo the Ring. Gollum, however, has been captured in Mordor and isn't released until 3017. Aragorn catches Gollum around this same time and takes him to Mirkwood Forest to stay with Thranduil and the elves there. Gandalf is busy studying the history of the One Ring in Gondor, and Gollum escapes from Mirkwood in 3018 (presumably before Gandalf has time to meet with him).
So, it may seem strange that Gandalf is in Lord of the Rings: Gollum since their paths don't seem to directly cross during this time. However, no designs for Aragorn have been shown, so if the game deviates too far from canon, it's possible Gandalf may work as a stand-in. Given his long life, Gandalf (sometimes by other names) is often used as a continuity device in Tolkien's works, so Daedlic may be using a similar approach here.
Most likely, Gandalf will have a bigger role in Lord of the Rings: Gollum than he did in the books during this same time period. Because he's working behind the scenes so much at this point in LotR, he may only make a few cameos rather than stay central to the game's plot. It really just depends on how much Lord of the Rings: Gollum sticks to what's canon. Either way, fans should still be excited that the wizard is appearing in the game.
Sources: The Lord of the Rings/Appenix B, Lord of the Rings: Gollum/Steam
