The symbols \$G\$ and \$H\$ are typically used for the forward gain and feedback fraction in negative feedback block diagrams like this:
The choice of \$G\$ is, I suppose, an obvious one, meant to be mnemonic for gain.
But \$H\$ I don't get. Is it just because it's the next letter in the alphabet and maybe \$F\$ was already taken? Or is there perhaps a historical reason that might lead one to see it as a less arbitrary choice?
Note: I understand these symbols are not universally used in these roles. I kind of grew up on \$A_o\$ and \$\beta\$ myself, but as I understand it, \$G\$ and \$H\$ are more common in the control systems world and do have some advantages over other choices. Certainly conforming to a convention has its own benefits, whatever that convention is.