Hi, thanks for the question. I just added a documentation page for BondGate here:
Please let us know if there are some other docs that you need which are missing and we will try to add them if we can.
You can also see some helpful examples of how BondGate is used and can be used such as here:
Also, please note it's not strictly necessary to use the BondGate class to do gate-based / Trotter time evolution with ITensor. There's a tutorial code itensor/tutorial/05_gates/gates.cc which contains a nearly working example of doing such a time evolution method "from scratch" just using more basic ITensor functions and objects. The idea of the tutorial is that you are supposed to add the (two or three lines) of code that actually contracts each gate with the two-site wavefunction tensor, then test that it works. If you need help with the solution for that please just ask in a comment below.
Also if you want to post the code you are trying to make using BondGate, and have questions about that, feel free to post that too.