The Guardian model described in the Arcane Armor feature of the Armorer Artificer includes "Thunder Gauntlets":
Each of the armor’s gauntlets counts as a simple melee weapon while you aren’t holding anything in it...
If the Artificer holds tools for a material component of a spell, the description above doesn't apply. However, if the armor itself is used as the focus (assuming it's an Infused Item), the Artificer is effectively holding a weapon in each hand.
To deal with this, must the Artificer have taken the Warcaster feat (to allow performing the somatic component of a spell while holding a weapon)? Or can the Thunder Gauntlets be considered "sheathed" when not attacking with them?
Note 1: All Artificer spellcasting requires a material component, so performing a somatic-only spell is not a concern.
Note 2: I had missed the part where the "plain" Arcane Armor can be used as a focus, so the parenthetical above assuming it was infused is unnecessary.
My key takeaway from the answers: Although the Thunder Gauntlets "count" as simple weapons, the Artificer doesn't hold them. Therefore the Artificer's hands remain free to use for somatic components and the Warcaster feat isn't necessary.