Definitely have a go, but manage expectations down and apply for others.
We were in exactly the same boat last year (DD's and our #1 choice, offer looked unlikely given not top 10-15% of class). We told DD that "yes, if you get in, you can go here", which helped motivate her to practice (not easy!) - and I made sure to print the old tests with the G&L logo on the front... But also "it is a really very popular school with 900+ girls who want to go there, so the school has to disappoint most of them - so we should not be sad if it does not work out"
Another reason to try is that it is really difficult to predict how well they will do on those tests - at least until you practice on old NL group tests under timed conditions at home and see how it goes. Some girls will relish the "math" - where creative problem solvers can shine even if that talent has not been spotted by the current school - and observant readers similiarly in the English. I really believe test performance can differ a lot from current "levels"! Potential is different from current achievement.
In case comparison is of interest: DD got 3 level 5s (b/c) at SATS - she was probably coasting in her (state school) class and got mix of mostly 4A/5c in teacher assessments at end of year 6 - and mostly 4C at end of year 5. She was not selected for level 6 tests which was fine with us. Little tutoring and lax parents, and a bit lazy...
DD ended up leaving a lot of questions unanswered on the actual G&L test (so I knew hope was gone within a minute of picking her up, but kept my smile), but she did much better ploughing through the other NL "group" tests a week later, with offers from other schools (but not G&L). DD was OK about the test experience (liked the dolphin) which anyway was very useful practice. Now she is at a slightly less academic but still very good school, with shorter commute and a bit lower fees - probably a better fit after all. There were 3 from her school trying (you meet the others on interview day), but none made it, which also made it easier to accept.
The bottom line for me is that with hindsight we would have done the same thing and tried for G&L - no regrets. Good luck!