I wonder, how important is gap analysis for planning process in isms based ISO 27001:2013 ? Why we should do gap analysis (at this point, i want to make a plan for implementing isms in an organization)?
Although it is not mandatory by the standard, a gap analysis is important in a sense that it can provide you a snapshot of the current situation of an organization considering a referential (in this case ISO 27001). It can give you an overview of where you are already compliant with the standard and about the effort required to be fully compliant, allowing a better planning of an implementation project. If you already have some information security practices already implemented but you are not sure if they are complaint with the standard, performing a gap analysis can help you with this understanding. On the other hand if you are starting your information security alternatives now there is no need to perform a gap analysis (remember, this is not a mandatory requirement for ISO 27001).
so, it have no relation with establishing isms? i mean, i saw that some of reference just make gap analysis as one of its step, it placed between define scope and risk assessment. is there any method that make gap analysis as a step after we just passed risk treatment phase?
Please note that although Gap Analysis is not mandatory for ISO 27001, it is a good practice to perform it for the establishment of an ISMS compliant to this standard, for the reasons presented in the previous answer ("... It can give you an overview of where you are already compliant with the standard and about the effort required to be fully compliant...").
Gap analysis can be performed at any moment during your implementation project, but it is more effective before risk assessment (because its results can help you avoid unnecessary efforts and costs). Since you have passed the risk treatment phase (it is not clear if you already implemented the controls or not), the Gap analysis can help you confirm if your defined treatments that are the most relevant and proper ones to your context, or if you need to make adjustments (e.g., including, adjusting or excluding controls).