Just leave this place and return when you have gained some levels and an electronic lockpick (preferably Mk II).
That's not a very good suggestion, since a player normally obtains the Electronic Lockpick Mk II in San Francisco, which is near the end of the game. The Gecko Skinning perk will have worn out its usefulness long before then, so it's important to at least rescue Smiley as soon as possible, then return to crack the bunker at a later date. Gecko skins can be good income boost all the way up to the middle of the game and Fire Geckos.
OP, people have already given you most of the advice you need: Take the secluded path on the right of the map, waiting until the geckos are as far away as possible before moving forwards. If you're lucky, you'll only need to kill one or two of them as you rescue Smiley and bring him back to the entrance.
If you don't have enough firepower, go to the Den first and buy or steal an SMG or Hunting Rifle. You should be able to afford one, and if not, you can always murder one or both shopkeepers there, since no one cares.
If Sulik is dying, change his combat movement mode to "stay where you are" and/or give him some rubber boots. Failing that, make him wait next to the entrance so that there's no way for him to run into danger.
And if all else fails, you can try decreasing the difficulty. Doing Toxic Caves early on is actually one of the more challenging parts of the game, because of your low level and lack of expensive equipment and supplies. It's especially difficult if you don't have a "perfect" character build.