You will need to stop the application server itself and any node agents or other Java applications used by what you have deployed. If the installer has created Windows services then look for something that starts with "IBM" and/or "DB2" (if you installed DB2 locally on the same node) and stop them. Check task manager and look for any Java.exe as that will be WAS.
As you plan to uninstall if you get really stuck kill them via task manager.
You will need to remember to clear up the OS afterwards. There's no need to uninstall WebSphere, just the Gateway so please read ahead and work out what is required.
http://www-01.ibm.com/support/knowledgecenter/SSKTXQ_9.0.0/admin/install/gw802/i_rtc_t_stop_restart_single_server_i5os.html?lang=en
http://www-01.ibm.com/support/knowledgecenter/SSKTXQ_9.0.0/admin/install/inst_uninst_gw_produninst_ov_winux.html?lang=en