Gap statistics measures how different the total within intra-cluster variation can be between observed data and reference data with a random uniform distribution. A large gap statistics means the clustering structure is very far away from the random uniform distribution of points.

The number of clusters can be chosen as the smallest value of k such that the gap statistic is within one standard deviation of the gap at k+1. In your case, when k = 6, its value is greater than the value that k = 7 minus one standard deviation.

More details can be found http://web.stanford.edu/~hastie/Papers/gap.pdf

The link given below may be useful for Interpretation of gap statistics.

https://statweb.stanford.edu/~gwalther/gap

Please refer to https://arxiv.org/abs/1103.4767