Q: I've changed valve seals and all gaskets I'm getting gray smoke from tailpipe in 2000 Silverado

Gray smoke suggests burning oil but you can use a scan tool to look at fuel trims and see if perhaps rich operation is contributing to the problem as well. With regard to replacing the seals, if the valve guides are worn, that wear can overcome the seals and you will still get oil down the guide. Of course, oil can get past the rings too. To see if you ring wear, you can test each cylinder for compression, adding oil to any cylinders with low compression. If adding oil raises compression appreciably, you have worn rings. Another test is look for smoke under heavy acceleration; if it occurs that is indicative of a piston or piston ring problem. If you request an engine smoking diagnostic, the responding certified mechanic can determine what repairs will be required. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.