Prior to Immigration, in GRU RCC facilities have showers but they call the area "UA Arrivals" (if you know what I mean). Also you can go to Terminal 2, prior to Immigration by keeping on the corridor where UA lounge is and you'll find two RG business class lounges and one F RG lounge. I'm unsure whether they've showers. After immigration you've but the RG domestic lounge, where I don't believe there are showers.