Our 11 month old is starting to crawl, and it is beyond time to put a baby gate at the top of the stairs. We received a baby gate, along with instructions, which clearly state that the maximum acceptable gap on the sides and along the bottom is 3 inches. Due to the weird moldings opposite the gate and at the bannister post, we would have to mount this particular gate somewhat higher that we would like. The gap at the bottom would be approximately 4 inches. My wife, the mama lion and stickler for details, is uneasy about this, and wants to extend the gate downwards, somehow, to achieve a sub 3 inch gap. I am not ashamed to have two pieces of scrap plywood bolted to each other, but was looking for other ideas.
Measure your baby. The 3" gap is based on (a) minimum baby size across all babies expected to use the units, plus (b) a generous quantity of CYA on the manufacturer's part. Thus, 4" is likely just fine, and you can verify this by figuring out the minimum gap that your actual baby can squirm through, plus enough extra to satisfy you and your wife (reasonable and sane people) rather than lawyers who smell blood (and/or money).
If you're happy with a 3" clearance at the bottom, you could screw a painted or stained 1" x 1" strip to the bottom of the gate which might look nicer than plywood. If the gate is made of wood you'd use pointy wood screws to attach the strip, and if it's made of something else you should be able to drill through its bottom member and use machine screws and nuts. In that case you'd want to counterbore the bottom of your strip and size the screw so that its end, and the nut, are hidden inside the wooden strip to keep baby from getting scratched if he or she tries a breakout.
A baby gate typically has two wooden frames, that are allowed to slide past each other to modify the gate for differing widths of door that it will be blocking. The frames will be made of wood, at least in the baby gates I've seen.
My thought would be to attach a piece of plastic to the bottom rail of each of the wooden frames. A good choice for this plastic would be a piece of UHMW polyethylene, i.e., Ultra-High-Molecular-Weight polyethylene. It is not really hard stuff, so the baby won't scratch anything on it. It is also really slippery. A nice thing about UHMW poly is it is easily machined, cut it with a simple hand saw. Clean up the edges with a file, or even a pocket knife.
I'd find a piece of UHMW that is at least about 1/8 inch thick, and at least 2 inches wide. How to find something like this? It turns out that woodworkers use it to make jigs and various things. So you could buy it mail order, from a catalog that deals with woodworkers. Lee Valley sells a piece that is 1/4 inch thick, 4 inches wide, and two feet long. Cut it in half length wise, and one piece might be just enough to solve your entire problem. Even better, Woodworker's supply sells a piece that is 1/8x4x48 inches - perfect for your project. Woodcraft Supply also has the same size for sale, and you can even find stores for this vendor in your local area if you prefer, so you can go and see what the stuff looks like before you buy it.
Screw the plastic to the bottom rails of your gate. Do so by drilling a hole of the proper size through the UHMW. It is easily drilled as I said, although a little grabby. Select a screw that is the proper length to attach to the gate. If you use wood screws, make them JUST short enough to not go through the gate frame. I would also drill a pilot hole in the wooden gate, that is smaller than the screw diameter. This will stop the wood from splitting.
Better is to use machine screws to hold this together. Now you would drill holes through both pieces large enough to take the screw shank. You can use a countersink on UHMW to hide the screw head, or just use a pan head screw. On the back side, I would use a nylon threaded locknut. If you are careful in your choice of screw length, there will be no sharp screw thread that sticks out to scratch tender baby fingers. And because you use a locknut, no fears about these nuts coming off under finger tension.
With some care and pride in your work, you can extend your baby gate to be safe for the child, as well as looking as if it was professionally made to be that way.