Target 1 face-up monster your opponent controls; change it to face-down Defense Position. Once per Chain, when a Trap Card is activated while this card is in your Graveyard: Special Summon this card as a Normal Monster (Aqua-Type/WATER/Level 2/ATK 1200/DEF 0). (This card is NOT treated as a Trap Card.) If Summoned this way, this card is unaffected by monster effects, also banish it when it leaves the field.
Changes 1 face-up monster your opponent controls to face-down.
You can Special Summon this card as Normal Monster when a Trap card is activated (by you or your opponent). Gains immunity towards monster effects.
This card is not treated as a Trap card, therefore spell/trap removal effects does not work on this card.
If this card leaves the Field after it was Special Summoned, banish this monster.
Have weak ATK and DEF.
When you flip this Trap card after it was Set, it is still considered Trap Card, and it will only set a face-up monster your opponent controls, face-down. After that resolves, this card will be sent to the Graveyard.
After it was sent to the Graveyard, you can therefore activate its effect by chaining to another Trap card’s activation. Monsters set to face-down by this card effect, can be flipped face up again.
This card also acts as an easy tribute for Citadel Whale.
You can't activate this card manually from the graveyard, only if your opponent activates a trap card, you can chain this card's effect from the graveyard, and only once, you can't activate it twice in the same chain if you have 2 of these cards in your graveyard.
OP means floodgate is in a shitty box (generation next is pretty shitty overall compared to newer boxes), whereas Canadia is in a fantastic box with, like you said, geagias, amazoness support and masked heroes rolled into one
In a more competitive environment, Floodgate is a hit or miss. Almost everyone is aware how the 'BACKROWS' need to be wiped first before you attack, or if you can't, find a way to lock them. An advanced user WILL MOST OF THE TIME, take care of the backrows first before summoning anything. Otherwise, Fgate is better than Canadia if your opponent is lacking hard backrow-removal (witch, trunade,etc)
Continuing, Floodgate mostly screws those who uses monster effects as backrow removals, since they need to be summoned first. The same way between Mirror Wall and WoD, they're marginally the same but becomes better in "specific circumstances".
With Synchro summoning being introduced Canadia is better than Floodgate now. Gives you extra levels. If not Synchros summoning you get a card to tribute. Miles better in my opinion. Floodgate is still good though dont get me wrong!
Floodgate's effect is more powerful. But Yugioh is not always about power, Floodgate's activation window is very strict, compared to Canadia which can be done at anytime. Therefore Canadia proves to be more useful in more situations. The extra monster fodder is just cherry on top.
Palaeozoics combo well with the tiki trap monsters so if you crash with both tiki trap monsters on the field you destroy a monster then reset your trap