Straining to read text would be a valid extenuating circumstance. You would need to provide evidence such as a prescription from the optometrist dated after the test. A letter from the GP confirming migraines before the test would also be useful.
For a successful appeal, though, the academic evidence would have to be good. I'm not sufficiently familiar with the standard in Glos., but generally speaking the CAT scores are lower than I would like to see. (They're standardised nationally, so are not directly comparable with the 11+).
It's possible that the CAT scores were also affected by eyesight problems, and that your daughter has been disadvantaged to some extent whenever 'working against the clock'.
The G&T evidence may or may not be useful. I think that schools have targets, and within each school pupils are identified as gifted or talented on a percentage basis. The extent to which it carries weight, therefore, may depend on the quality of the school.
People with local knowledge are better placed than I am to comment on the chances of a successful appeal, but, in general terms, I think it would be worth having a go - despite the obvious stress - provided you can put up a strong academic case. See this link for some alternative indicators of ability:
http://www.elevenplusexams.co.uk/11plus ... rs.php#b11