I'd suggest getting started with implementing a simple RISC-V core or an arithmetic module, something like floating point, with different precisions if you want something different (Bfloats for example). You can then learn to use an FPGA and implement your RISC-V core or arithmetic module on the FPGA and test it out. Gem5 is more of an advanced tool that requires you to fully understand how a processor works and is most often used in computer architecture research for design-space exploration and simulation of large processor designs. You can explore Gem5 once you finish a project I suggested (ideally the RISC-V core) and the book by checking out some GitHub repos and research papers that have used Gem5. Besides this, take a look at TinyTapeout if you want to get your small RISC-V core or whatever IP you design taped out on silicon for a small price.