Gingivectomy or gingivoplasty problem, what's the difference?

Big difference. Gingivectomy is a gum surgery to eliminate all excess as well as infected gum tissue in order to treat periodontitis. Gingivoplasty is mostly a cosmetic procedure.
Removal for one. Gingivectomy is actual removal of some of the gingival tissue, whereas gingivoplasty is just a thinning or recontouring of the gingiva.