1 n. — predominantly Western Canada & Ontario, informal, slang, diminutive
underwear.
Type: 1. Origin — The term is a loanword from Eastern European languages (cf. Serbian and Croatian gaće, Hungarian gatya), originally brought to Ontario and Prairie Canada by immigrants. A survey conducted in Ontario and bordering regions in the US asked about the prank also called a "wedgie" (see meaning 2; see Chambers (1994-2004, question 74; 2012)). It did not elicit any responses from Americans such as gotchie pull or gotchie, the two most common responses from Ontarians (see meaning 2). US respondents exclusively used "wedgie". Thus, its origin is most likely Canadian. In British Columbia the term and its variants are much less widely known (Chambers 1994-2004). However, the oldest quotations in our database are from British Columbia (see the first two 1987 quotations). Chart 1 shows that gotchies is almost exclusively used in Canada.
The regional dimension cannot be tested with the Frequency Index method due to low or no hits for most variants in the provincial domains. Among them, only gonch figures prominently enough. This term, however, shows a clear western Canadian and Northwest Territories provenance (see Chart 2).
See also COD-2, which marks the term, s.v. "gotchies" as "Cdn slang" and as a diminutive of gotch (s.v. ibid).
2 in compounds such as gotchie pull — informal
a prank where the perpetrator pulls up on the back of the victim's underwear.
Type: 1. Origin — See etymology under meaning 1. The standard variant of gotchie pull is "wedgie" (Chambers 2012), which is a North American term.
Images:

Chart 1: Internet Domain Search, 18 Jul. 2012 
Chart 2: Regional Domain Search, 30 Jun. 2016