You have to become human at the bonfire in order to see summon signs. If you die, you become hollow and can't summon until you reverse hollowing again.
There are a lot of humanities available throughout the game, so you can feel relatively free to try summoning for the boss a few times. When I play the game without doing anything special with humanity (dont use it but also dont farm for it), I usually end up with around 60 humanities by the end of the game just from treasure pickups or the odd loot drop.
Also, if you kindle the bonfire while you're human, you will get more estus so you wont die as easily. The limit is 10 estus at first - the firelink shrine bonfire has this unlocked by default, so you might want to use the elevator to rest there before fighting the gargoyles. Later you can find an item that lets you kindle to 20 estus.
If you're worried, there's no downside to just trying the boss a few times until you know what it does. Upgrading your weapon helps a lot too. You should ideally have a +5 weapon for the gargoyles.