Hi Ashu,
Sorry for the delay in response. Attached are the GBC sequences for our PMDC screen. These GBCs were not designed specifically, rather they are a random sample of a random oligonucleotide pool.
In general, 15-20 bases will be sufficiently long that there will be an extremely low probability of observing GBC sequences that are within one or two mismatches of one another. If you are planning on designing GBCs it is also worth avoiding sequences containing restriction sites for subsequent sgRNA cloning (but this will not occur very often).
Hope that helps!
Atray