Apart from compulsory grades you don't have to pass any grade to do the next but there are some minimum ages for different grades.
Alot of regions now have a national 5 so gymnasts may do that instead of club 6 or 5.
There are handbooks on the BG website. It changes often so I won't link . The pass marks for compulsory grades also often change.
You can only do one club, regional or national grade a year but you can do 2 compulsory grades a year.
If you do club, regional or national grades you can compete Classic levels at the end of the year (zink, copper, bronze etc)
If you do compulsory you can compete voluntary levels (5 to 2)
You can swap between the systems at the start of each year but not during the year. eg if you compete national 4 you can't do vol 4 at end year and if you do compulsory 4 you can't do copper at the end of the year.
All the above competitions are organised regionally. Some lead to a national final and some don't.
There are also lotsof invitiationals competitions throughout the year. Some are grades based, some voluntary levels based and some do their own criteria. So something for everyone really in abundance.
I like the fact you have grades in spring with set skills followed by levels in the autumn with choice of skills. We don't have any set floor or beam routines just set skills to include. Vault, bars and range routines for grades are set.