This kind of question comes up a lot here.
Do not add grains to the boil! That's true in barley-based brewing as well as gluten-free brewing. Adding grains to the boil will cause all kinds of bad things to go into your beer--starch, grain dust, tannins, beta-glucans, etc.
Now, you can steep gluten-free grains, prior to the boil, at temperatures below 150°F. However, unlike in normal barley-based brewing, where you can get specialty caramel malts that contribute flavor and fermentable sugars when steeped, in gluten-free brewing you can't buy caramel malts (or malts at all, really), so steeping the grains will NOT add fermentable sugars. You can get some grain flavor and some color, but if you steep too hot or use too much unmalted grain, you'll end up with an excess of starch in your beer, which will create haze and may add off-flavors. Unless you're planning on doing a secondary fermentation with Brettanomyces or some other wild yeast or bacteria that can eat starch, in which case, go for it.
If you want to use grains, it's best to use no more than a couple pounds per 5 gallons, unless you want to a) malt them yourself and do a minimash or make your own caramel malt, or b) you want to use an enzyme formula containing at least alpha-amylase. If you're steeping unmalted grains, they will not affect your gravity or ABV, so do not reduce the amount of syrups you're adding. And again, it's important that you keep the steeping temperature LOW, as unlike in doing a mash, you do NOT want to gelatinize the starch (aka "bring the starch into the brewing liquor").
If you just want to add body, you can get good results with both lactose and maltodextrin. I've never used lactose, but I've used up to 8 oz of maltodextrin in a 3-gallon batch, so I can safely say that a pound in a 5-gallon batch is just fine. I'd recommend starting there, and try increasing it further if it's not enough.