A: To know whether you need GXP or BB30 chainring for your SRAM crankset, you should be aware of the type of bottom bracket (BB) installed on your bike. The BB type name is usually imprinted on the crankarm. However, in case it is not visible, you can measure the BB spindle diameter. GXP bottom bracket has a spindle diameter value of 24mm and BB30 - 30mm.
If you have a GXP BB installed, you will need a GXP chainring, and if your bottom bracket is BB30, then BB30 is the chainring you should opt for.
Also, modern BB30 bottom brackets' vary in width. Choosing GXP or BB30 chainring is up to the width of your BB spindle too. We made a simple scheme that will help you define what chainring to pick depending on the width of BB spindle on your bike.