I have similar issues coming up - DC1 is 7 and was flagged as G&T in reception, I hadn't quite understood his ability to learn things wasn't 'normal'. He's in a class of bright children and so doesn't stand out that much, and may well have 'peaked' early, others may well catch up/over take him in the next few years.
DC2 is just about to start school in September and is no where near when DC1 was at that stage - it's hard to remember she's not 'behind' - she's where she should be.
Going on I can see there being issues with DC2 finding things hard that DC1 found easy (DH made a comment last night that we will have to teach DC2 to read, rather than just leaving her with some books and then her figuring it out by end of reception - he said this after both were in bed so we're careful not to let the DCs hear us) - but we see this as our challenge to manage.
We have the added challenge of living in an area with the 11+ DC1 was looking at some test papers last week (the little girl next door is doing 11+ in September and showed them to him) he found it easy now, so we're assuming he'll probably pass without any tutoring etc, but then it's going to be harder to 'hide' the difference in attainment between them if DC2 doesn't (and unless she really blossoms accademically when she starts school, it's unlikely she'll be in the top 25%).
Knowing the neighbours are going through it has made us think about what we'll do in that situation, and we've decided if DC1 does pass the 11+ as expected, there are single sex grammar options around here, we'll pick one of those so that DC2 couldn't go to the same school anyway, and avoid the "why aren't you going to the same school as your big brother?" questions when she's at that stage.
Put against that, DC2 is more phsyical than DC1, she walked earlier, ran earlier, she could use a scooter before turning 2 (DC1 took until 3.5 to get that), she could ride a bike without stablisers before 4, she's good at football, throwing and catching, etc and her pen control is amazing - she's drawing much better than DC1 was at pre-school age. She may well just have different abilities.