It's not against the terms of the GPL to place any requirements on people at installation time. After all, they can get the source and can remove those requirements if they want. That said, there's no good reason to force them to accept the GPL, although continuing to show it to them would be a useful way of reminding/informing them of their obligations re distributing the software, and may help with public awareness.

Thanks for info. The next version won't require user to accept the licence.

WinSCP will not install unless you select "I accept the agreement" when presented with a copy of the GNU GPL. However, the GPL clearly states in section 5:

5. You are not required to accept this License, since you have not signed it. However, nothing else grants you permission to modify or distribute the Program or its derivative works. These actions are prohibited by law if you do not accept this License. Therefore, by modifying or distributing the Program (or any work based on the Program), you indicate your acceptance of this License to do so, and all its terms and conditions for copying, distributing or modifying the Program or works based on it.

If you do not intend to modify WinSCP, or to distribute WinSCP or its derivative works, then you are not required to accept the GPL. It is against the terms of the GPL to force the user to accept the GPL when installing WinSCP.

Section 7 of the GPL states:

If you cannot distribute so as to satisfy simultaneously your obligations under this License and any other pertinent obligations, then as a consequence you may not distribute the Program at all.

This may be taken to read that the combination of the GPL license which states that you don't have to accept the GPL, along with the requirement to accept the GPL in the installer for WinSCP, means that WinSCP cannot be distributed at all!