Majtki - undies, panties
Gacie - colloquial term for any type of garment to CYA... Sometimes it sounds derogatory, when talking about the old and worn ones (stare podarte gacie), sometimes it sounds really cute when talking about new elegant dress pants
Portki/ pory - colloquial term for any type of pants/ very large or derogatory
https://www.scholastic.com/teachers/books/monsters-love-underpants-by-claire-freedman/ look it will be easy to understand ;) in UK we call this part of underwear simply pants as Jellei wrote in his post.