1995 gmc sierra ,front pads changed now dash brake light ON and spongy brake pedal

Sounds like you have a combination of air in the brake system and/or low brake fluid level. Not sure how you bled the brake system, but I would do that first. To bleed the brake system of air do the following. Have an extra person to help. One person in the truck and truck running. Have this person pump up the brakes until you get the most brake pedal you can. Hold the pedal down firmly while your helper releases the bleeder on the caliper. Hold the brake pedal down until they have the bleeder closed firmly again. Do this on both front calipers until you get a solid brake pedal again. Two things to remember keep an eye on the brake level as you do this so not to draw air into brake system. Also be sure to wait to release brake pedal during bleeding until your helper has the bleeder screw tight again.