Removing old fashioned saddle from seatpost
I found that the GMC Denali 21 speed uses an HL Aluminum Micro Adjust 27.2 X 300mm, so I assume your seat is very similar to those found on the Denali. In this case you will need a small crescent wrench that will fit between the guts and the side of the saddle. Some bolts are square and will stay put when loosing the nuts, but some are not and might require an opposing wrench on the other side. You only need to loosen each side a half turn and you can slide the seat off the post. The problem is that you will only be able to replace it with similar guts. You might be able to swap out the saddle itself if the rails match, but my recommendation would be to switch the whole saddle/post with a more modern set.