ok, so some people wrap their children in towels, so they can't move their arms, but you need two people one to hold the child one to do the drops, you can pin the child this way (if you only have one adult to hand) kneel over them without placing any weight on them and then put the drops in.
I personally would never do this, as it is setting the child up for a horrible experience.
I take the child sit them on my lap, scoot them down so mainly there chest is on my legs, my arm around their body holding it up to my chest, and then use my other hand to hold open the eye and put the drops in, the first one is easy, they are prepared for the 2nd one.
Or you could lie your child on a changing mat, and proceed to put the drops in, as said before the first will always be the easiest, as they are not expecting it, so alternate eyes each time so you can hopefully get something in each eye.
Personally i think gel is harder then drops, but you may think the other way round.
Some parents apparently put in when they are asleep but this seems cruel to me.
Just yesterday i put drops in, the child was sitting at the table, i held their head back opened they eye and put them in, obviously it does depend on the child.