I've been using Ubuntu 18.04 since release and never had this problem. However, I've installed gcc-10 previously for my own work, which might be why I never had this issue.
I think I might have an idea of what this is. Sometimes when packages are built for Ubuntu 18.04 in frameworks like Appveyor, the libgcc-s1 library is added as a dependency. Ubuntu 18.04 by default does not have any repository that contains this library as it seems to be part of the newer gcc-10 compiler package.
I'm not sure if GitHub Actions has this same issue. If this is indeed the cause, simply adding libgcc-s1 as a dependency will not work. It is already present in the dependency list which is also why the build fails. Instead, the above repository would have to be added earlier on like in a pre-install script.
I am leaning towards Michael's suggestion of waiting to see if any others come up with the same issue.
@Micheal, you might be better off adding the above repository and installing gcc-10 from it, since you'll be able to get updates. If you do plan to remove your installed gcc-10 package and re-install it from that ubuntu-toolchain-r repository, I would love to know if OpenVSP installs without issues after adding the repository before re-installing gcc-10. APT should pick up that libgcc-s1 as a dependency and install it by itself once the repository is available.
- Cibin