The Proteas had raced to 117 for one as England’s fast bowlers struggled to utilise the new ball after AB de Villiers won his first toss as South Africa’s Test captain and elected to bat.
But Steven Finn finished with 50 for two – he was unlucky not to find further reward from a tremendous spell either side of tea – and Ben Stokes chipped in with a couple of wickets as England put themselves in a strong position to land a series-clinching victory at the Wanderers.
“When you lose a toss and you have the opposition seven down you’ve done all right,” said Gibson.
“We struggled early on to find the right length I think, and then after that I thought Finny was outstanding the way he bowled and we had to keep going.
“They [South Africa] played quite well, they left well in the first 20 minutes, half an hour. Perhaps we didn’t find the right lengths but, as the day went on, we got better and better.”
Gibson revealed that a more aggressive strategy had paid dividends, as England struck four times in the final session before being frustrated by an unbroken eighth-wicket stand of 42 between Chris Morris and Kagiso Rabada.
“All the time we’re assessing the lengths, trying to find what is the right length to bowl on every given pitch we play on,” he added.
“I didn’t think that we found that length in the first half an hour – we spoke about it, we said that we need to risk a drive.
“The guys worked really hard and seven down is a plus for us on that wicket – it looks a really good pitch.”
Finn returned from a two-year Test absence during last summer’s Ashes and, after missing the tour of the United Arab Emirates against Pakistan through injury at the end of 2015, has now overtaken Dane Piedt as the leading wicket taker in this series.
“He’s been outstanding since he came back into the team,” Gibson said.
“He’s come back in and shown what he can do and gone from strength to strength.”