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During her entire career, Gibbs's record against Top 10 names is 0-5, and she took a single set. If we take a look at how those matches went, then we have results like these:

Serena – 26 16
Kvitova – 26 46
Kvitova – 36 46
Kvitova – three sets
Muguruza – 16 06

If we include Top 11 - 20 players, then it looks like this:

Pennetta – 46 06
Svitolina – 36 67
Wozniacki – 16 06

It's clear that Gibbs has problems with strong names. On a margin of 18.5 games, she went into an under 5-3 times. However, we have to look at some things from another perspective and against whom she didn't go. Twice against Kvitova. The Czech knows she's not the most stable player in the world. She's always prone to oscillating and giving games away. She didn't go against Svitolina who wasn't a Top 10 level in 2015, and she's not Top 10 now either, and at the time she was Top 20. Muguruza left her at a single game last season, and this season Woziacki did the same, Petkovic at four. Not a single player in even slightly important matches can fit into the same sentence as Serena except Serena herself, and their H2H was played in 2012, so I'm not giving much thought to that match. But, if someone manages to get a game against Woz this season, four against Petkovic, to lose to Muguruza last season with a single game, then it's clear we're talking about a lack of quality.

Even if she wasn't at 100%, Serena is above all these names, and relevant matches clearly indicate that Gibbs has problems with even weaker players.
Since that desperate day she didn't have since 204 and losing to Brengla, Serena didn't lose outside of Top 50 names. If we count all of her matches from last season and this one against Top 66 onwards (Gibbs is 92nd, and I took the 66th place on purpose because Gibbs never did better than 68th) Serena went into an under of 18.5 in a record of 7-3. But last season it was 7-1, while this year she didn't go under on the first tournament of the season where she looked really bad. Parmentier and struggling with her, and the defeat by Brengle. But, again, besides a bad tournament that can happen to anyone, and all are names stronger than Gibbs. She remained at 19 games against Bencic and Safarova who are realistically now Top 20, and until recently they were Top 10 and injuries held them back. That's just one games less than what we need here, and quality-wise they're two classes above Gibbs. Next to this first tournament of the season, Serena's record in covering the under is 7-1, and mostly they were players stronger than Gibbs. If you ask me, this margin is at least one game too small.

Against Safarova Serena recorded 15 aces and won 84% of the points after the first serve. Safarova and Bencic could stood up to her with strength (up to a point), and they were left at 19 games, and Gibbs whose game is based on defense and footwork won't be able to stand up to her. This under can only be knocked down by Serena.