These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Marty Stu: A king, partly divine, all but invincible, Super Strength, gets laid with every girl in his kingdom but snubs the goddess of R3D H0T S3XX0RS... Gilgamesh is wish-fulfillment Snark Bait by modern standards. He's effectively protected by a Grandfather Clause, since he predates most tropes.
Unbuilt Trope: A lot of the wish-fulfillment elements are portrayed as immature character flaws that needs to be overcome (consider how later Classical Mythology might have treated the same subjects), his heroic adventures only sets of a chain of events leading to the death of his best friend and when going on the most important quest of his life what do he do? He fails.
Memetic Badass: The sole reason why the story survives today. We wouldn't have so many tablets if they weren't being made and put all over Mesopotamia just to tell everyone how Badass he was and how you can never hope to reach the level of sheer badassery he was at.
Fridge Brilliance can be applied here when you figure that well, though he may have failed to achieve literal immortality within his own story; the Epic of Gilgamesh in the meta sense has become immortal and survives to this day.
What an Idiot: If you have a plant that will make you immortal, don't wait until after you've taken a bath to use it.
He was actually going to give it to an old man to see if it would turn him young again, so he was probably playing it safe since he didn't know if it would actually kill him instead.
So basically, not only was Gilgamesh not an idiot, he was actually incredibly Genre Savvy! Impressive, considering the age of the tale.
The real problem wasn't whether Gilgamesh was Genre Savvy enough, it was that he was Wrong Genre Savvy. Which might be understandable since he was the first to try to subvert many foolish actions, but because he was the first, he didn't realize option 2 could have just as many problem as option 1. For example, he knew it was bad luck to fall for Ishtar, but he didn't seem to realize just brushing her off and rebuking her was just as bad as accepting her advances.
Woolseyism: Since some portions of the story have been lost, some translations feature original material to fill the gaps which occasionally works quite well. Most notably, at least one changes Gilgamesh's motivation from wanting immortality for himself to wanting to bring Enkidu back to life.