Grainger said: "Anna wasn't back to full fitness [in the run-up to the regatta] but we just wanted to get some good racing.
"The fact that we're still ahead of the field even without the preparation under our belts, it's looking good from here."
Women's pair Glover and Stanning continued their rapid rise to prominence, which brought a surprise world silver last October.
"It's a really good feeling, we're really pleased with our performance. But at the same time it was a really close race so we're not there yet," said Stanning.
Glover admitted: "It's a bit surreal still and I want to keep hold of this momentum. Twelve months ago I would have aspired to be where we are now so this is justification for the hard work we've put in."
There were silver medals for women's lightweight double scullers Hester Goodsell and Sophie Hosking, and the GB men's and women's quadruple sculls.
The men's eight, featuring 1992 Olympic champion Greg Searle, were pipped by the Netherlands in a sprint for second place behind world champions Germany and there was also bronze for the men's double.
GB performance director David Tanner said: "It was fantastic to see the women's pair to kick off the regatta as they did when they are still rookies really.
"The women's double, because of injury, is still a work in progress and to win was significant.
"The open men's four were outstanding in dominating their field and what a sub in Peter Chambers to help the lightweight men's four win by their biggest ever margin at this level of event."
Only two crews missed out on medals: the women's eight finished fourth after a tough route through qualifying for the final.
And Olympic champion Mark Hunter came second in the B final of the lightweight doubles with stand-in Adam Freeman-Pask, as his usual partner Zac Purchase is ill.
Single sculler Alan Campbell also missed the event through illness.
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