Just a quick disclaimer, I do not have a GF but I have spent much time working on different processes.
BeerSmith is set up to be pretty universal in basic process variables which are common to every brewer's process. It is also very versatile in allowing the user to adjust the profiles as needed to accommodate their particular brewing style.
For the GF, I would recommend starting out with a single infusion mash profile. If you are mashing in for a 66C saccharification rest, then mashing out at 75C you will have a single infusion mash with a mash out step. Are you doing no sparge? Pouring your sparge through the basket? Or batch sparging in a separate pail/kettle/container?
I would take a single infusion mash profile for medium or light body. Copy it and give it a new name. I like to number mine so that they immediately jump to the top of the mash profiles listings and are easier to find. Now open the profile to edit it. Set the grain temperature, Tun temperature (your GF), and sparge water temperature to reflect your typical conditions. Since you are working with the GF and most likely will be recirculating your mash liquor through the grain bed, I would set the GF temperature to be your desired mash temp, since you will be allowing the GF to control the temperature.
Now click to highlight the mash step in the dialog box and either double click it or click on 'edit step'. Change the mash step temperature to 66F. Now, here is where you need to know your system limitations. Set the infusion volume ('water to add') to the recommended value for the GF. David Heath has a series of GF videos that are well worth watching and he discusses volumes in this video: https://www.youtube.com/watch?v=vBjD124jkIo
You can double click on the mash out step and set the temperature you want there. Since you will be using the Grainfather to control and raise the temperature, this is just a temperature step and you may need to change the step type in your mash profile from 'infusion' to 'temperature'.
If you are not sparging, then click the BIAB mash with full boil box and enter in an estimated number for your initial volume at the start of the boil. The program will adjust this number when the profile is used in a recipe, but it gives you a quick look at the volume of the mash versus the capacity of the GF. From my understanding, the GF will only do about a 14 liter batch if you do no sparge brewing. Again, I would refer you to some of David Heath's videos on the GF as a guide.
If you are going to pour through or fly sparge, then you don't need to check anything, it is the default setting for the mash profile sparge step. BeerSmith will calculate an approximate volume for you to use in your sparge automatically. From my few attempts at fly sparging, I would prepare excess water and sparge until you reach your desired pre-boil volume.
If you are going to do a batch sparge, and I know some people do this in a bucket with their GF basket, then click the box for 'batch sparge using batches that fill' and you can set the sparge amount for whatever your bucket or kettle will hold in relationship to the GF.
I hope this helps. There are several videos and guides on the BeerSmith YouTube channel and if you search it you can find several others set up videos for the GF and BeerSmith profiles which can help with your set up and adjustment to match your particular piece of equipment.