Just tightening should work unless ends are damaged. I would never use sealant on an injector line. 1) Pressures of at least 1800psi. 2) If the sealant gets into the injector it could cost alot of money to fix.
Depending on injector tip, if the sealant plugs the injector nozzle it could blow apart and put hole thru piston. Seen this with water in fuel system with unit injectors. Not pretty or cheap.
tightening may help, but if not, the lines have an o-ring on the inside that needs replacing. Go to your local hardware store and get a new o-ring, or for that matter, replace all of them. cheap insurance. The old ones are probably rotted stiff, remove them with a hook pick, then install new ones coating them with diesel fuel first.
Not all that familar with the GM engine. On the 6.9, its easy to over tighten the pressure line and cause problems. On the 6.9 there is no o-ring on the pressure line. its a flare fitting nipple on the line and a seat on the injector. Try unscrewing the line about a half turn, then tighten it again. Sometimes they just wont seat and you have to replace the line. It doesnt take much of a defect to cause a leak at the injector. On the return side of the 6.9 injectors, you have a plastic "hat" that slides down over the injector. Under that HAT are O rings. If they leak, replace them, if they still leak, you will have to replace the plastic piece too. Go to your local International dealer and get a kit. Costs like 35 bucks and will have new plastic hats and all the o rings.