I don't have an HFGH, but if it has a base plate, logic dictates that it needs to be secured to some type of base that is itself, securely fixed to the ground.

I believe that most folk use treated 4 X 4's anchored to the ground and if that proves your best choice, you need to prepare the ground under the foundation properly before you lay them because the least bit of ground shift or heave to cause the structure to go out of plumb will result in warped frames, doors and vents that won't operate and panels that crack or pop out of their frames.

For that, start with stakes, twine and a leveler to get your foundation dead level. Once you have established a level line, a gravel filled narrow trench under the perimiter where the base will lay would help to prevent rotting, as it will allow drainage under the timbers.

Once the timbers are laid, the easiset way to secure them to the ground is by drilling through them and driving rebar stakes through them, long enough to reach a bit past your frost line. If you can, or have someone who is handy with woodworking to help, the best corner joints would be half laps, secured with long lag screws.

The only other option would be a true floating foundation, in which the base of the GH is bolted to a heavy timber frame that just sits on the ground, so if the ground heaves, it would shift the entire structure with it, but not warp the framing.

I've read so many posts here by people whose small, lightweight GH's have had panels pop out and doors jam and have come to think that the real reason for it has been that once dead level bases shifted just enough to warp the frames out of plumb. It doesn't take much.

BTW: They are a bit pricey, but may well prove worth the extra cost over wood later. Instead of treated wood, check out composite timbers. They are perfectly straight from the gitgo and once secured into a frame to fit the base of your GH, will never warp or rot.

I would not drill holes through the treated timbers this Will allow water to enter and the treated part is only on the outer surface not all the way through.
Here is a link that may help.

Here is a link that might be useful: Mods to HFGH

No harm at all in drilling though the timbers. Just fill the holes with silicone before driving in the rebar, countersink them and the excess silicone that oozes out the top will seal the holes. Then, when it dries, just slice off the excess to leave a flat surface. We experimented with a piece of scrap and found that the silicone sealed the hole at the bottom as well as the top.

However, I do agree about the treatment, although it depends on the mfg. A timber that's been properly treated was completely cured before being treated under pressure and will be treated to the center, because when it "refuses" the treatment solution has completely replaced the moisture originally in the wood.

But they aren't the ones you will find at the discount home centers and why, for a permanent installation like the base of a GH, I've come to believe the composites are a wise investment. In fact, if you compare prices on really high quality timbers vs. the composites sold at most home centers, they aren't too different.

BTW: Even with the composites, sealing the holes is still a good precaution to prevent water from entering the holes, and possibly freezing and causing them to heave. It doesn't take more than a bit of the sealant though, because the hole should be just large enough to allow the rebar to fit, but small enough to require some pounding with a heavy mallet to get it through and down.