Spray a 9x13 baking dish. Cook pasta until about 2 minutes under regular cook time (probably about 5-6 minutes). Drain, add to <g class="gr_ gr_76 gr-alert gr_gramm Grammar only-ins doubleReplace replaceWithoutSep" id="76" data-gr-id="76">prepared</g> dish.
Combine evaporated milk, Ranch dressing and pepper in a <g class="gr_ gr_72 gr-alert gr_spell ContextualSpelling multiReplace" id="72" data-gr-id="72">3 quart</g> heavy saucepan. Bring it just to a boil then lower heat, stirring frequently until dry ingredients are dissolved. Simmer, uncovered, 25 minutes, stirring frequently. <g class="gr_ gr_69 gr-alert gr_gramm Grammar multiReplace" id="69" data-gr-id="69">Sauce</g> will thicken slightly.
Layer pasta, chicken, sauce, and cheese into <g class="gr_ gr_73 gr-alert gr_gramm Grammar only-ins replaceWithoutSep" id="73" data-gr-id="73">dish</g>. Bake uncovered at 350' for 25 minutes or until hot golden brown and bubbly. Remove lasagna from oven and sprinkle 1/2 cup gluten free bread crumbs on lasagna, return to oven and brown the bread crumbs for 5 minutes.
Sprinkle 1/4 cup green onions before serving.
Let rest 10 minutes before serving.