There never was a gasket, OE used epoxy.
There is a lot to be gained from opening one up, if for nothing other than to make sure they are free from vacuum leaks.
The poor two-piece intake has gotten a bad rap over the years, but I have found them superior to even the "Big Iron" 16-Horse intake. The Two-piece has fewer bends, and those fewer bends have a more subtle transition. If you know how to play with fuel distribution on a "wet" flow bench, you can unlock it a little, to the point of having to run a leaner jet, which translates to greater efficiency, and port velocity does not become such a huge requirement. On the log-style intakes, you had to have port velocity just to pick the fuel back up once it rounds the bends, so to speak. But if done right, you won't have the inherent over-rich cylinder two problem.
If you've never played with any of that, start with this...tack your gasket onto the block perfectly, separate the intake manifold, bolt the bottom half up, and match the block and manifold to the gasket. Now flip the block over, bolt the top half on and do the same. You can, if you feel you need to, leave a couple of those small screws in place to keep the halves matched, bolt it as an assembly to the block, then remove the other half, but I don't feel that's necessary.