I have recently built an addition to my 50+ year old ranch home. During the addition plumbing for a 3/4 bath was added to the existing plumbing. An abandoned dry well was destroyed during construction because the contractor did not see anything being led to it. My laundry machine drains into an ejector pump which pushes the grey water up and out to a leach field (I think, as the ground is always greener there and the snow melts quickly in that spot). Over the past year, we have experienced the kitchen and new bathroom toilet gurgling and spitting up water whenever the washing machine releases it's water load. Additionally, the ejector pump spews some of the gray water onto the basement floor. This seems to get worse when the ground freezes.

We have hard water so we use very little laundry soap but at times the suds overflow onto the basement floor was well. It appears that the laundry and new bathroom vent through the new bath roof. Can someone please tell me why this is happening? And why the toilet and sink in old part of house does not gurgle? Thank you

Over the past year, we have experienced the kitchen and new bathroom toilet gurgling and spitting up water whenever the washing machine releases it's water load. Additionally, the ejector pump spews some of the gray water onto the basement floor. This seems to get worse when the ground freezes.
You have a partial blockage in the drain line that produces back pressure and a bubble of sewer gas when the washer discharge hits it. This is the "spitting and gurgles" The solution would be to snak and clear the drain line. That's one line but you have another problem. The ejector pump backing up. This makes me wonder if the leach field isn't getting clogged to the point of not accepting the volume of water the pump puts out. If the lines aren't below the frost line in your area then the lines are subject to freeze up.

why does the toilet and sink in the old part of house does not gurgle?
Because the bubble of sewer gas doesn't get back that far. It comes up in the closest fixture. Good luck, Tom