Using Gigaplant's effect like that. The whole summoning stuff back from the grave?
I've tried to pull off similar shenanigans in the battle phase on duel network and people were not amused; it's only supposed to be done during the main phase, unless it's a Quick Effect, as far as I'm aware.
Oh, I see, and I apologize for the ambiguity; Gigaplant hasn't been activated. Supervise and Chirubime are activating simultaneously. You are correct regarding Gigaplant's effect, though. Ignition effects (basically, things that are manually activated) like Gigaplant only can be activated in the Main Phase unless otherwise stated on the card itself. Trigger and trigger-like effects, whether optional (Chirubime, Botanical Girl, and Rose Archer) or mandatory (Supervise), are activated when their triggers are met, unless they
miss the timing. Continuous effects are always active so long as the condition behind them is met (usually being face-up on the field, but Gemini monsters differ slightly here as outlined in their effect). Basically, what's happening is an interaction between the following three bolded effects:
Equip only to a Gemini monster. It is treated as an Effect Monster, and gains its effect(s). When this face-up card on the field is sent to the Graveyard, select and Special Summon 1 Normal Monster from your Graveyard.
This card is treated as a Normal Monster while face-up on the field or in the Graveyard. While this card is face-up on the field, you can Normal Summon it to have it be treated as an Effect Monster with this effect:
● Once per turn, you can Special Summon 1 Insect-Type or Plant-Type monster from your hand or Graveyard.
Your opponent cannot target face-up Plant-Type monsters for attacks, except this one. If this card in your possession is sent to your Graveyard by your opponent's card (either by battle or by card effect): You can Special Summon 1 Plant-Type monster from your Deck, except "Chirubimé, Princess of Autumn Leaves".