The head gasket has round holes in it where the coolant flows through it around all of the cylinders......the BLOCK has irregular shapes BELOW the head gasket where those round holes are located.......SO......when you remove the head, the head gasket usually separates and leaves a certain amount of gasket material over the holes in the engine block, therefore those passages look like they are "clogged" to a certain degree.
Also, if you have recently bought the truck, then the previous owner may have known that the engine had a blown HG before he/she sold it and put some kind of head gasket sealer in her! I just recently bought such a truck and pulled it down and performed the appropriate repairs to her........and she is happy again!
Here's a few pics showing the some of the HG material and how it looks like it is blocking the coolant passages.....
And here you can see how the underside of the HG shows where the passages in the block are larger than the holes in the HG.......
After the block is cleaned up you can see that the passages in the block are a lot larger than the passages in the HG....
And the new HG....
Post up some pics of it all if you can do that.........as you can see here......it helps a lot!!