What type of gin pole are you intending to build? Moving pole? Stationary pole? Self contained (moving or stationary) pole (having its own integral winch)?
Decide what you want. I wanted to have a light weight strong portable self contained gin pole that can be used on land or water. So I built a self contained stationary pole gin pole with a yolk on the bottom that simply slips over the mast bolt on the tabernacle. A big U on the end of a 6 foot pole equipped with a winch and a block.
A gin pole foot can be as simple as a notch cut into the bottom of a 2x6 that generally fits the mast with a strap around it if you like. It doesn't' necessarily have to fasten to the mast - it can just rest there if you like but its more stable fastened. Once loaded the gin pole system holds it in place.
Lots of different ways to do the same thing - no one right way.