sc-inc-gpc0 action is used with http-response, with means it will not be evaluated when http-request silent-drop is executed, because no response will happen.
You may want to execute the sc-inc-gpc0 action on http-request as well:
http-request track-sc0 src
# increase gpc0 if getting blocked
http-request sc-inc-gpc0(0) if getblocked
# in either case, silently drop the connection
http-request silent-drop if getblocked or isblocked