I could be wrong, but this is my understanding of the gemtech bcg and really any adjustable gas block:
The BCG has an "On/Off" switch so that you can switch from normal gas flow to reduced flow for suppressor use. I don't think it's adjustable, just on/off.
Adjustable gas blocks allow you to tune your gas flow, but they aren't as quick to adjust. Who wants to go "can on, turn screw in five turns, can off, turn screw out five turns". Set it and forget it.
So yes, they achieve the same basic ends (reduced gas flow in the BCG) through different means. Using them together allows for a greater amount of tuning (basically lowing the gas flow to allow the bho to latch reliably in cold weather) but why? Unless you plan on shooting a mixed schedule of suppressed and unsuppressed I don't see any reason to buy the gemtech bcg. Leave the can on, tune your block for your preferred ammo, and go shoot.