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.
This is the core of a computer engineering degree.
You're looking for (1) probably a first course in the basics of circuits, (2/3) a first- and second-course in "digital design" or "logic design", (4) an operating systems course (emphasizes how this stuff all works with "The real world"), a basic programming class if you don't have one, and some domain-specific stuff such as digital signal processing, graphics, statistics, finance, or whatever else you're trying to do with this thing.
Probably 5-6 good university classes in total. If you could do these classes a la carte, it's probably the fastest way to learn this stuff (skip the gen eds and all other non-degree requirements). Frankly, it's a lot if you have no prior experience, but I'm not sure that's true.
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.