Dear Gaoshuai,
The ElastoDyn module of FAST / OpenFAST does not output strain directly; rather, ElastoDyn can output deflection at a node. The term “strain gage” is used simply to mark a node in ElastoDyn where response outputs (deflection, sectional loads) can be output.
The definitions of TwrGagNd, etc. have been discussed several times on this forum, e.g., see: fore-aft and side-to-side accelerations at mid-tower - #4 by Jason.Jonkman.
Best regards,