It's cool, this stuff is a bit hard to wrap your head around at first.
Lots of rack units contain both limiting and gating functions. To see what I mean, just go to the "live sound" or "pro audio" section of any big web store like MF or Sweetwater, and click on the links for signal processors and then compressors. Look at the 1-rack-height units with more than 4 knobs.
You connect them by wiring the preamp output (or line out, or effects loop send) of your amp to the input of the comp; then wiring the output of the comp to the power amp input (or fx loop return) of the amp. Your bass plugs into the regular input on the front if your amp.
Another option is a "channel strip" which contains a preamp, some EQ, and comp/limit and gate functions (depending on the specific unit--they don't all have the same features). These can be used as described above, or you can plug your bass straight into the preamp, to bypass your amp's preamp stage.
