Other than posting in the wrong area, you are more than welcome to ask here.
Before I go on about DarkRP, are you using ULX? Is it not showing you/logging player deaths already?
You can set the echo to be admin only for ULX.
If it's not, then, as in many other functions done badly in DarkRP, and one of many reasons I dislike DarkRP, it's preventing the hook that monitors death from letting any other script work.
What you're trying to do shouldn't be overly difficult if DarkRP uses the hook for player death.
Would be a great way for you to learn Gmod lua if you don't already know it.
Find the hook. Edit so that the check for rp_showdeath = 0 portion has a check for admin that continues with an admin only player message.