It's all online, nothing like an appeal, you won't be face to face
120 is so near the qualifying 121 mark, that you stand a good chance.
The onlu caveat is your child really needs to be working at GDS (not EXS) and the heatecher gave them two good score for grammar suitability (so a 1.1, 2.1, 2.2). The scores are given to the council before the test, so they cannot be changed retrospectively.
It's a simple process, you fill in a form, the head gets a short form to fill, then you attach the head's bit with your application and submit.
You upload any documents to back your case. For example the last few years school reports, showing your child was working at GDS.
Do not include any extra-curricular activities (such as sporting achievements or instrument playing), as it's based on academic attainment only. Also it's not an appeal, if you succeed your child will be considered qualified for any grammar school. Sporting or instrument playing, etc, is only considered at appeal for a particular school, which a SR is not.
Also include any extenuating circumstances, that show why your child didn't pass, but very briefly, as you are so close. So don't labour the point, and only include it if there are any,botherwise it can hinder, rather than help.
Do not include illness as a factor, even with a GP note, as it cannot be considered unless the school were aware before the test.
Do speak to the head,vso they know you are doing this and can look out for the form.
You are only one point away, so really the proof of consistent attainment, for the last few years, is all you really need.