While it is very possible that Gamon was just bullshitting to have an excuse to look from afar at Nagatoro and Senpai's date, I do feel there's a chance she also has their best interest in mind.

Since it's implied that Gamon has been good friends with Nagatoro for a while, there's the chance that 1. She's never seen Nagatoro in a serious relationship, and is afraid she could make Rookie mistakes that might result in throwing off Naoto OR 2. She's been on dates before and screwed them up.

Either way, there's the possibility, at least in a Gamon that is unaware of Senpai's discussions with Sunomiya, that Naoto might be apprehensive or won't know how to react with a very forward, non-teasing Nagatoro. I don't think Gamon is trying to cock-block as much as trying to make them succeed in the long-game with the limited information she's got.

Or she's just bullshitting for shenanigans, either way, 3 weeks cannot come fast enough.