The Evil Within

You should be able to get this before mid-way in the game if you put all your green gel into a gun. My suggestion for which to get done first would be the revolver pistol. You'll get a lot of ammo for it and it's a good all-round weapon to start with.

However I got this achievement after completing the game on Casual without using any green Gel for the 'It Is What It Is' Achievement.

Upon starting New Game+ I had accumulated over 350,000 Green Gel which I then used on various upgrades. The first available time to upgrade your character is in chapter 2 in the cabin mirror.

You should make a save before doing any upgrades and select them in turn. For example you could get the Guns all done, re-load the checkpoint and do the Stock and then Attributes. Then when you're ready to replay the game pick and choose what you want/need.

BILLTHECRUSHER has also pointed out that the easiest/cheapest weapon to upgrade is the crossbow. He makes a good point

The easiest and fastest weapon to upgrade is the crossbow. It only takes 3 upgrades for 3 options. I unlocked it by chapter 5, even after wasting some gel on health and stamina beforehand. It can come through gameplay and replays but this is the quickest option.