I'm not sure how programming FPGAs compares to GPU programming, but it's a completely different way of thinking compared to traditional software.

The last time I really did FPGAs was over 10 years ago, but unless the tools have gotten orders of magnitudes better, in addition to the other concepts mentioned you need to understand clock domains, metastability, pipelining, etc.

I preferred VHDL to Verilog because you shouldn't need a lint checker for your HDL. But unless it's changed, Verilog is a lot more popular.

Training is available, for example https://www.xilinx.com/training/atp.html . It probably costs more than you're interested in paying, and you should have the basics covered first.