I went from a 93 GMC to a 94-98 GMC. You need the grille, side markers, reflectors, bumper fillers, bumper brackets, and probably some of those square nylon screw nuts. The 94+ grilles "snap" into the bumper filler on the bottom, and then the filler piece itself is what gets bolted in.
Chevy grille 15981106
Chevy side markers 5977737 and 5977738
Chevy reflectors 5977459 and 5977460
Bumper fillers 12376285, 12375170, and 12375171.
My center bumper filler (12376285) did not fit at first because the chrome bumper sat too close to the truck. The 94+ had longer bumper brackets that pushed the front bumper out 1/2 inch or so more so the wider bumper filler could fit. So I swapped all 4 bumper brackets out from a 97 silverado. They are 15686152 (discontinued) 19417107, 15686149, and 15686150. The screw nuts are those square shaped, yellow(ish) nuts that the grille bolts thread into. My truck was missing a few so I bought some off eBay.
Let me know if you have any more questions.