I am an 11+ tutor for glos.
It is definitely possible, the 11+ forum give the impression that you need to tutor hard for a year, you really don't.
Glos has grammar schools in Cheltenham, Gloucester and Stroud. The one in Cheltenham, Pates, is really hard to get into and by the sounds of your dds maths, I wouldn't bother.
The ones in Gloucester and Stroud are of a similar level, so she has chance.
Normally I would say, if they are working at greater depth in English and maths, then you need to do practice papers and they stand a decent chance.
Your problem is that she is not at that level in maths.
You will need to work hard between now and September, she may be able to make up the gap, she may not.
Buy the CGP books, not the 10 minute practice books, but the A4 size practice books, these ones:
books
work through them. She needs to be able to do all the maths in the books, and be getting about 75% of the questions right.
Then the assessment tests at the back are at the same level as the actual test, to get into schools in glos she needs an average across the subjects of about 65% or more.
NVR, VR and Maths are in the test (VR is basically English and includes comprehension etc)
You are working towards the CEM test, NOT the GL test, and don't be sidetracked by things you find online with loads of fiddly Verbal reasoning, you don;t need it for the CEM test.
Vocabulary is key, if she is well read and articulate, she probably has good enough vocab. But consciously adding more words to her vocab over the next 3 months is worth doing.
It doesn't cost to register for the test, and you can withdraw at any time, so register, think about it and work on it and then decide.
PM me if you like.