G0 commands are for rapid moves and typically, (in my experience at least), get issued without a feed rate - simply going as fast as the machine is configured for. They can take a feed rate though and that’s how I’ve been testing my machine. Any G1 command would likely have a feed rate associated with it, which of course should/would likely be different for X/Y moves and Z moves.
I’ve just taken a quick look through the config file and don’t see a separate default for Z, but there is a gamma_max_rate. If you set this to 120 and left the default_seek_rate and alpha/beta_max_rates at 24000, (or whatever works for your machine), the lower gamma rate will prevent the machine running the Z axis at the default_seek_rate.
I just noticed that there are also x/y/z_axis_max_speed configurations too. Not sure which of these drives what, but I’d set them the same as the alpha/beta/gamma_max_rates absent any reasoning as to why not to.
I don’t have a Z axis on my machine yet as I’m still in the process of building it, but I tested this out by setting my beta_max_rate and y_axis_max_speed to 120 and issued some G0X__Y__ moves. X went at full speed and Y crawled along at 2mm/sec.