Q: My opponent has an Imposing Sovereign, and my devotion to blue is 4. If I cast Thassa, God of the Sea, does she enter tapped?

A: Nope! This seems to confuse a lot of folks, so we'll go step by step through it.

In any zone other than the battlefield, Thassa is always a creature. So on the stack (which is the zone she'll enter the battlefield from), Thassa is a creature spell. But she's a weird sort of creature spell: she has an ability that can cause her, once on the battlefield, not to be a creature. And that's where things get tricky.

Imposing Sovereign's ability creates a replacement effect, which modifies how certain permanents (creature permanents) enter the battlefield. Replacement effects, as you'll recall if you read CI regularly, apply before the event they're replacing happens. So if Imposing Sovereign's effect is going to apply, it will apply before Thassa is on the battlefield.

This means that, before Thassa enters, we need to figure out whether she's going to be a creature for Imposing Sovereign to apply to. The rules tell us (specifically, rule 614.12 tells us) what types of abilities and effects to consider when we're figuring that out, and that's a pretty short list, but it does let us look at Thassa's own ability. But at the time we're asking what her ability would do, your devotion to blue is only 4 — it won't hit 5 until after she's on the battlefield, and we're doing this before she enters.

Which means her ability answers back "devotion to blue is only 4, I wouldn't be a creature in this situation", and Imposing Sovereign's effect doesn't apply. So Thassa enters the battlefield untapped, and then as soon as she's on the battlefield your devotion is 5 and immediately she's a (still untapped) creature.

The Gather Specimens replacement effect is applied before any other replacement effects that would also modify how the creature enters the battlefield. These are usually worded "as [this creature] enters the battlefield" or “[this creature] enters the battlefield with.” For example, if your Gather Specimens has resolved, then the following things are true: -- If a creature with devour would enter the battlefield under an opponent's control, you choose and sacrifice your creatures as it enters the battlefield under your control. -- If Voice of All would enter the battlefield under an opponent's control, you choose a color as it enters the battlefield under your control. -- If Clone would enter the battlefield under an opponent's control, you choose which creature it copies as it enters the battlefield under your control. -- If a Wizard would enter the battlefield under an opponent's control and that player controls Sage of Fables, the Wizard will not enter the battlefield with a +1/+1 counter on it as it enters the battlefield under your control. -- If a Wizard would enter the battlefield under an opponent's control and you control Sage of Fables, the Wizard will enter the battlefield with a +1/+1 counter on it as it enters the battlefield under your control.

614.12 Certains effets de remplacement modifient comment un permanent arrive sur le champ de bataille. (Voir les règles 614.1c-d.) De tels effets peuvent venir du permanent lui-même s'ils affectent uniquement ce permanent (par opposition à un sous-ensemble général de permanents qui l'inclut). Ils peuvent aussi venir d'autres sources. Pour déterminer quels effets de remplacement s'appliquent et comment ils s'appliquent, vérifiez les caractéristiques du permanent tel qu'il existerait une fois sur le champ de bataille en tenant compte des effets de remplacement qui ont déjà modifié la manière dont il arrive sur le champ de bataille (cf. règle 616.1), des effets continus engendrés par la résolution de sorts ou de capacités qui ont changé les caractéristiques du permanent sur la pile (cf. règle 400.7a), et des effets continus des capacités statiques de ce permanent, mais en ignorant les effets continus de toute autre source qui devrait l'affecter.

The Gather Specimens replacement effect is applied before any other replacement effects that would also modify how the creature enters the battlefield.

ce que tu cites ne traite que d'interaction d'effets de remplacement, aucune mention d'effets continus de type la dévotion.