The trick is to be gentle and use a little technique. I sit behind them and tuck them between my legs so they don't try to back up.
First, I show them the tablet in my hand so they can see what they're eating, then use the magic word "eat" as I point at it in my hand a couple of times to make it clear. I always use the same command "open your mouth" as I open my mouth too, as I pry theirs open, and place it as far back as I can without making them gag. Once it's in, I close their mouth and hold their head up while I gently stroke their throat until they swallow. And, of course, praise them for doing a great job, no treats necessary.
The way I see it is that I'd also be freaked out if a giant forced me to swallow something without knowing what it is. They get it and get used to the drill. You only have to be nice and clear about it!