頑固【がんこ】
stubborn, obstinate, pigheaded
頑張る【がんばる】
to persevere, to persist, to keep at it, to hang on, to hold out, to do one's best, to insist that, to stick to (one's opinion), to remain in a place, to stick to one's post, to refuse to budge
頑丈【がんじょう】
solid, firm, stout, burly, strong, sturdy
頑強【がんきょう】
stubborn, dogged, persistent, tenacious, tough, sturdy, hardy, strong