You should be asking the school to meet your child's needs whether he is gifted or not. (What did his profile look like? Flat across the four areas, or noticeable spikes?)
If he is struggling with certain areas, school need to be addressing them. Given that school are aware he is bright, and have offered extension groups in the past, I don't think there is any necessity to start parading the report and demanding he be given additional stuff just because you have a report, however, it would be interesting for the senco to be able to read the report on terms of the areas he struggles with and the discrepancy between his potential and his lack of ability to display this in certain areas.
Fwiw, at 6 it is on no way unusual for a boy to be finding lengthy writing and creative writing dull as ditchwater, and to be unable to bother with it...
What other tests did the psych do to rule out ADHD and asd? I would expect a full battery as well as big standard wechsler and WIAT. Did they do Conners etc? BASC? A gifted score on an iq test doesn't rule out additional dx (ds1 has the same profile, but he also has an ADHD dx and aspergers traits. Dd2 is gifted and has cp. dd1 is gifted and NT).
Ds1 has never applied himself as much as he should, and at 6 was still largely ambidextrous.
he's still gifted.
At 6, I wouldn't fret too much about the writing side. If he can articulate the story etc, you could ask him to type it, or you could type it. You can also try one or other of the zillions of writing schemes, or follow a programme for kids with fine motor issues (as these are often the root cause of handwriting issues or reluctance). Search in here for mrz's ideas wrt fine motor skills.
I like 'write from the start' which is an OT programme (Teodorescu and Ali available through the dyspraxia foundation) but tbh, the writing issue for a 6 yo boy is absolutely typical. Being gifted doesn't stop him from being a 6yo boy. 
If he is desperately unhappy with the level of work, then sure, go back in and ask to discuss. School seem to recognise he's bright though, so I don't think this is a case where you have anything to 'prove'.
Did you get him tested because of the possible AS, btw? Or for another reason? What were the results of the ADHD/ AS tests? Which ones did they do?