I actually found out the solution after writing this message. --p-pseudocount is already added in coorelation clusteringand also some other features of gneiss.
so basically if anyone wants to run qiime gneiss correlation-clustering --i-table table.qza --p-pseudocount 0.5 --o-clustering heirarchy.qza.
similarly this can be used in dendogram-heatmap and other 3 or 4 features of gneiss.