The play on the name of the Finnish truck manufacturer "Sisu" comes from the Finnish Candy brand that goes by the same name. The original official Fraktur font logo of the candy brand reads '𝕾𝖎𝖋𝖚', but is spelled 'Sisu'. The first letter S (𝕾) looks like G, and the second s (𝖋) looks like f, making Gifu.
The Gifu Brand is also present in the Finnish-made game INFRA, on Sisu-based trucks seen throughout the game. This was by coincidence, as both INFRA's developers and Toplessgun came up with the joke on their own at the same time.[1]
The cap that the Uncle wears has a Gifu logo on it.
In the older versions of the game, there was a logging Gifu that did not appear in game unless it was spawned in by the player with the command "Sisu" which is a Finnish automotive company that the designs of the Gifu were based on.