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I'm writing a scene in which for characters play a high stakes poker game. So far my narrator has been an omniscient third person, who just does not wish to enter into the characters heads.

I started going around the table. I have the fingers fumbling with the corners of the cards, the tapping of cigars on the greasy tablecloth and the occasional sip of liquor. I even added background crickets and the smell of freshly cut grass. Then I slowly uncovered the hand, and went on with the bidding. It feels gimmicky. Dull.

Do I need to show the characters thoughts in order to get some believable tension? Can it be done with a mere description? Is it the timing, or perhaps I'm not focusing on the right elements?

In one question: how to slowly and credibly rise the tension using pure description (no dialogue) as a poker game unfolds on the table (two rounds at most)?

You don't need to show the characters thoughts to know what they are thinking. Well written body language should be enough. Instead of portraying standard card playing behavior, give your characters individual tells and gestures.

Player A sips water instead of liquor.
Player B twists his wedding band, it's missing a stone.

Player C keeps staring at Player D's well manicured hands
Player E is allergic to fresh cut grass, he's rubbing his watering eyes, trying to stay focused on the game.

Put things in the setting that are unusual, but not to distracting. If they are playing in private room in a classy casino, there might be a crayon sitting on a side table. One of the characters notices it, then looks back at his cards.

If you are playing in a greasy bar, get rid of the greasy table cloth, make it pristine white. Then when you tap your cigar on it, let the ashes spill across it.

How are you characters interacting with each other? who are they avoiding eye contact with, who are they smiling at?

You can use pacing to help hold the tension. Keep you sentences short, but not choppy. The more specific you are with your descriptions the better the imagery will be.

What you describe in the question sounds perfectly fine to me. So if other people are telling you (or you're telling yourself) that it's not quite right, the problem isn't what you're doing, it's how you're doing it.

After all, some people love to watch TV coverage of poker games. There the narrator can only describe what the camera sees, and maybe add some outside information (like a player's history). There's no way to know what's happening in the players' heads.

While adding in POVs from different characters might work great, it could also be very confusing and busy and turn the reader off. Do it only if there's a strong reason for it and not because you're worried the narrator you have is too dull.

For those who like watching poker (not being one of them, I can only guess), I assume they get to know the players some. The way that watching the Olympics is more fun if you know some of the backstories of the top competitors (how he slipped on the ice half a second before the finish line 4 years ago and lost the gold, how she trained in a country that doesn't even get snow and still made the Olympics). But pausing the action to stick in one more "heartwarming" story is enough to make you barf (don't do this).

So get to know the characters ahead of time (I don't know your story so I am guessing you do this but I don't actually know). We need to know what's at stake for each character as s/he plays. Reputation? Money? Proving something? or? Then each change in the cards and bets has meaning.

The sounds, smells, sights, and physicality of the scene you describe help set the mood. Not just for the reader but for the characters. You can also show what some of the characters are feeling by their actions and what they do and don't notice. Distracted, nervous, focused.

When you're done writing it, show it to someone you trust to tell you the truth. See what does and doesn't work and write it again.