I would add a caveat.
If she is at one of the girls 11+ preps in West London who expect the majority of their pupils to go on to SPGS/G&L/Francis Holland etc, I would not bother unless the school told you she was a borderline candidate and could do with a bit of an extra push in one subject or practice in exam speed etc. There is a lot of history and trust and the school report will be important. If the prep says that the schools you mention would be a stretch then they are unlikely to give an unqualified recommendation.
However others a bit further away or co-ed, may not have the same focus on the very competitive West London do-or-die 11 plus, particularly if the majority are heading elsewhere eg boys 13 plus, Dulwich, grammer, girls boarding (Wycombe Abbey, Cheltenham Ladies etc.) The schools will need to keep up the subject teaching and so have less time to polish VR and non-VR exam skills, and your child might need some extra help with preparation.
Even so you dont want to do too much, as the process is fraught enough as it is and you want your child to remain calm and perform on the day. Buying a batch of papers and having your child get used to format and timing so they can go straight in and get on with it should be enough. And do get hold of as many past papers from the relevant school as you can. The school will have covered all the maths and English they need. The time might be better spent on extra curricular activities which build confidence and give a child something extra to talk about at interview.
Both the schools you mention are academic. SPGS is very full on and a borderline child might be better off higher up the ability range elsewhere. G&L is a lovely school with a reputation for nurturing and some super pupils, but it is not the only choice in an area whose schools are completely over represented in the League tables and where choices include LEH, Putney, Wimbledon, NHHS, Frances Holland and Queensgate, as well as the scope to transfer to either Kings Wimbledon or Westminster at sixth form.
If your child is more maths/science than English it is worth considering Latymer Upper. They have only been taking girls for about 6 years, and selection has proved much harder for prep schools to predict. They are hugely oversubscribed, so it is quite possible to get SPGS and not LU. Their maths exam has a reputation for being tricky (and is worth practicing) and they may take girls whose have something specific to offer but who are not the perfect all rounder. (In contrast an all-rounder might prefer G&L and the fact they offer IB.)The presence of boys and their great science facilities means the school has a real edge for girls who fit this profile. In addition their sports, music, art and drama facilities are easily as good any of the girls schools.