tongue is made up of thick mucous membrane
,so it has got more tendency to bleed.
after stopping blood from the wound there is formation of clot.while gargling clot is removed and bleeding restarts.
in your case this is the reason after gargling you get bleeding from the site.
you stop gargling for few days till your wound is healed.
if pain and swelling is there go for one course of antibiotic.
ok n bye