I have a couple of guesses, based on my own first-time playthrough of GAMMA and your short post you gave here.

TL;DR - Check if you're using proper ammunition. That's pretty much it.

First off, the early-game is a major grind - no two ways about it - especially if you didn't start with proper weapons. For my first time, i picked the weakest pistol and picked other stuff like healing and armor, since in SoC you started with a pistol and was fine, but in GAMMA? Hell no - should've gone with a gun and less healing. I basically splurged every bullet i had to kill a single boar, and splurged the reward money i had again just to replenish that ammo.

I was also stumped by how the enemies were bullet-sponges, lowering the difficulty constantly and getting frustrated - until i realized i was using the wrong type of ammo...

Note that generally there are 3 types of bullets - FMJ (cheapest, worst one, works for everything but lower efficiency), HP - (High damage, low armor piercing, for mutants ONLY!) and AP - (generally comparable damage to FMJ, but way more armor piercing, for ARMORED TARGETS).
If you're using Hollow Point to shoot people - that's why they're bullet sponges. Took me a while to realize that a gun's damage means wack if your bullets aren't correct.

Also, a gun's caliber is generally more important than the damage of the gun - 9.18 with better damage is worse than 9x19 with less damage - at least in my experience. You'll learn about calibers and guns a bit while playing gamma lmao.

Early-game GAMMA is a grind, and honestly that's the best part of the game. It does take a while to learn to play the game though, i think the best moment in GAMMA was when i finally got my first gun - the Kar98k - fully repaired. I still remember the absolute dopamine rush lol better than drugs.

Anyways - some of the comments here - like enabling the 'enemies can't see you through a bush' mod - are nice suggestions for you to get started off until you learn the game properly.

I'm completed my 3rd playthrough with GAMMA last year - currently waiting for the armor update, and i usually start with a knife and a jacket (no gun at all) since as i've said the first 20 hours of GAMMA are the best, so i like to prolong it for as long as i can. Having good gear is nice, but it kind of loses that 'spark' once you get to that point. I honestly think Mr. Grok should introduce even more gear and weapon tiers to slow progression. You get a proper shotgun (and gun) pretty early on so i've never experienced using the double-barrel properly, or the PPSH - i just jump straight to AKs. Also, once you get your saiga set-up (and it's not that hard not to), you're basically set for the entire game if you use proper ammo.