Sewer gas from a laundry tub trap?

Could someone tell me what sewer gas smells like from
a laundry tub trap?
I installed a laundry tub and put a trap under drain about 6 months ago. It is in my garage on the opposite wall where the washer is in the house. The washer drains down a line and I drilled a hole and put the line from the laundry tub into it.

I just noticed a smell (it smells like mildew clothes when you forget to take they out of the washer) in my garage and thought an animal died and seached garage and cannot see anything so I thought it might be from the laundry trap. This smell just started about a month ago that is why I cannot understand why it should start smelling now if it is coming from there.

The line from the tub goes down on a slight angle and I can see water when I look into the sink.

Drilling a hole to make a connection in a drain line is not the way to do it! My guess is that this is leaking gas. That connection needs to be both water and gas tight, and there must be the proper pitch from the pipe. Last, and as important, there must be a vent, to allow this sink to drain properly without siphoning the water out of the trap, and to get the flow proper.

But what does the gas smell like.

Have you ever driven by a sewage treatment plant? Used an outhouse that needed some care? Think mouldy, sometimes sulfurous, nasty smell. There can also be some methane - smells similar to propane. The methane (rarely) can build up to where it can ignite. Methane as in fart...all sorts of things get put down the sewer - it gets diluted, decomposes, but...