Hello Yuksel,
See this snippet from SAP Note 376148 -
"With Basis Release 6.10, Transaction ST03N replaces Transaction ST03. All dialog response times displayed in Transaction ST03N include the GUI time. The difference dialog response time - GUI time gives the corresponding dialog response times without the GUI part. The analysis view 'Response time distribution' can be used to display the distribution of the dialog steps with or without GUI time.
Also, see the attached screenshot from one of the text books.
As per these the GUI time is included in response time.
Cheers,
Pankaj Kedia