I installed a new retaining lock on the fuel line going to the sending unit.and now its leaking gas
what do I need to do to stop it

 

get a bucket. and don't put in any more fuel in your tank until you get this resolved.

you probably have put it on wrong or lost a seal off the end of the pipe and didn't notice it was lost

So - it's a male-female connection, high pressure per say, has seals. The locking doesn't prevent leaking it only keeps the two pipe ends together (the connect is "designed to prevent maintenance" as you've noticed - to force you to go to the dealer or buy a special tool - a seedy practice).

I assume you already know how to safety remove fuel pressure so it doesn't squirt in your eyes, and that your light source should be a mechanics spark-free light, and etc.

If you dented the ends you need to use the proper tool to straighten them.

If your missing a seal keep in mind: it must be a gasonline/oil resistant seal not just any seal that fits.

 

I thought of something else. An old truck will have rusty lines old gaskets.

If it's rusty it will never seal: forget it, install new lines.

Gaskets. Old gaskets, especially fuel ones, can quickly expand and become ruined when exposed to air. Once that happens there's no way for you to re-seal them. (when working with gas-holding gaskets, keep them wet with gas or not dis-connected very long - else you'll need new gaskets)