Hi Troy,
there are two kinds of ready-goes: tiered and non-tiered.
Tiered ready-goes:
For those RG, the tier is explicitely written in the name of the ready-go (see for example here for a legend ready-go and here for a tour legend ready-go). For those RG, only players from that specific tier can play
Non-tiered ready-goes:
This is the main kind of ready-go (no tier written in the name of the tourney). For those RG, any player from any tier can enter and play.
Hope this helps,
Regards