I have just been through the process this year and have another sitting next year. My advice would be this:
1. Remember that the person who knows your child best is YOU and you are one of the best resources they will have. Your commitment to the process is, in my opinion, vital. The fact that you are here speaks for your commitment! Explain that this year will be tough going but will give your DC that all important choice in where they go to school. Get them on board, its essential believe me.
2 BUILD VOCAB: Start now reading childrens classic literature. Buy a book as 'their dictionary' and add all new word, meanings and some antonyms and synomyms. Play word games such as Scrabble/Boggle etc. Discuss the words and meanings. Use more complex words in everyday chat explaining what they mean. Soon they will become common place. If you have time devise some quizzes/games of your own for word groupings (types of trees/animal babies/colours/boats/aircraft etc. etc.) and get your DC to sort into categories. This is fun and they enjoy it.
3. MATHS: they MUST have a good grasp on the basics. So lots of mental maths practice for speed (I kept this very informal, just totting up prices in the shops etc.) Drill in those times tables so they know them as a matter of course. Add, subtract, multiply and divide.
4. CODES: Practice makes perfect here. Make a list of all the types of codes and tips and tricks. If you intend to work with your child you need to be as well versed in all the question types as your child is. They will look to you for guidance.
5. PRACTICE: I did short/quick fire papers once or twice a week and a long paper at weekend. In the last two months we did back to back papers at weekend to simulate the day. Felt cruel at the time but was very beneficial. Talk through the wrong answers.
6. SPEED: this comes with practice and familarisation. I often challenged my DS to a duel. See if he could beat me on speed and accuracy. I did go a bit slower to let him win. Being 'competitive mum' was likely not helpful! This really does help. Also some kids prefer to start with certain types of question to get their brain going. Leave out long questions to the end. They take WAYYYYYYYY to long and are only worth one mark (like all the others).
7. GUESSING: If in doubt ....guess. Preferably an educated one of course. In practice papers I got DS to write EG (Educated guess) above the ones he had guessed. That way I knew which he got right and which he was just lucky on and could go back and explain.
8. TUTORS: I have used one with DS1 and she offered a different teaching style from me. She was mainly comprehension focused and he enjoyed going. DO NOT fall into the assumption that if you have a tutor you will not need to do anything else or that it will deffo mean your DC will pass. My DS1 is bright and I tutored him at home (and external tutor) because I wanted to do everything to make sure he passed as grammar is the right place for him (leaving nothing to chance!). Tutors are up to you, some do some don't. They are NOT essential and you can do it yourself :0) with dedication.
9. MONEY: This all sounds expensive and it can be. There are lots of excellent resources around some of which are not appropriate for the Glos exam. Chose wisely and go for quality. There are other strands explaining which are best to use. You can find appropriate free resources online. Of the shop bought my personal favs are: Walsh, Bright Sparks and IPS (I think that's the name!) They are quite challenging and will need to be worked up to but offer excellent prep.
10. STAY CALM: Now I am the last person to be giving advice about staying calm!! This year I am determined to keep it real as long as I can. Getting in a panic offered nothing last year. It was hard keeping it to myself in front of DS and so this forum became my ranting/sobbing ground. It has been excellent support and the source of many pieces of useful info.
11. Yes anything can happen on the day. However, if your child is going in confident they have done the work, well and rested and with their 'game face' on there is less liklihood
of it all going pear shaped.
Getting into grammar is not everything but it can become obsessional. Have a good back up plan if all does not go according to wishes. I am not sure how much luck I should wish you (what with my DS2 also in the mix!) but I do hope you find the process a bonding one. I know I did.