I also have a small HFGH and keep my plants in pots of assorted sizes. I have 30g bins in my larger Rion GH for peppers and tomatoes. Nothing gets planted in the ground itself.

A trough bed either side of the gh might be one way to do it.
The compost is above the ground and contained so your gravel is safe. The water run off from the bed can be directed into a container for recycling. The best thing is they are very simple and cheap to make.
I`ve found it impossible to overwater and plants grow like crazy.
Lets see if i can get a pic to appear which might help :)
{{gwi:295932}}

The trough bed is interesting. Because there is air space under it, do you find it loses heat in the winter? That would be my concern.

How is that constructed exactly?

Hi
The frame is simply a rectangle of 3x2 timber with four legs cut from a 3x3 fence post.
The light green material is a waterproof tarp material forming a loop. It acts as a gutter to direct the run off to a catchment container and also to shade the root zone.

The black material in the pic is a geotex weed barrier type fabric which allows air and water through but prevents the roots and compost going anywhere.
It is free draining and allows plenty of oxygen to get to the roots from above, below and the sides.

I haven`t had any problems growing stuff in it over the winter so i guess heatloss isn`t a big factor.

Cherry tomato plants love it..the bed is underneath these somewhere :)

{{gwi:295933}}

Is the black fabric touching the colored tarp, or is it suspended above it?

I started out with one raised bed in my greenhouse this year, and liked it so much I've added another on the other side. I left my floor open so I'd have unlimited soil depth, and simply did a standard type of raised bed, like a person would do in the garden. I live in a fairly mild climate, however, not so many concerns about frozen ground. I do plan to grow many greens this winter in those beds, we'll see how it goes!