Compiler 'riscv64-linux-gnu-gcc' not found

How do I open GCC compiler in Linux?

And that the difference between the two is that riscv64-unknown-elf-gcc targets bare metal and uses the "raw" RISC-V ABI while riscv64-linux-gnu-gcc targets RISC-V running linux and, for example, uses the System-V (rather than raw RISC-V) ABI..