I've seem a lot of people telling that greenboard drywall SHOULD NOT be used in bathroom and wet areas. However, in Brazil (where drywall is kind of a novelty yet) the manufactures recommend greenboard for bathrooms and describe treatments to make it safe to use (even with tiles), not only that, but cement board is very difficult to come by here.
Basically, it says that the junction between the wall and the floor must be waterproof treated up to at least one feet up the floor (using various methods). Shower areas should be completely waterproof using the same technique, and after that you can install tiles using a flexible grout and mortar on top of the waterproofed drywall.
My question now is: is it safe to use greenboard if done in that (correct!?) way?
Green board is basically sheetrock with a waxy paper face. It's not as stiff as cement board, nor as waterproof, both of which are nice to have when applying tile.
CAN you use it? Sure. It's been done before. Is it to code? Not sure. You'd have to check with your local authorities.
If you decide to use it, see if you can find a product called 'redgard'. It's a paint-on waterproofer that should help keep the green board dry.
I'm not sure if it's available in Brazil, but if you can get it - I'd recommend using Schluter Systems. They have a product called Schluter Kerdi that can you even install over drywall in a shower installation. I've used it here in states on a bathroom remodel. Here's more info about it:
Product Page - http://www.schluter.com/8_1_kerdi.aspx
Product Faq - http://www.schluter.com/6478.aspx#kerdi
The codes have changed on this in the fairly recent past. In 2000, International Reseditial Code (IRC) had this section.
R702.4.2 Gypsum backer. Gypsum board utilized as the base or backer board for adhesive application of ceramic tile or other nonabsorbent finish material shall conform with ASTM C 630 or C 1178. Water–resistant gypsum backing board shall be permitted to be used on ceilings where framing spacing does not exceed 12 inches (305 mm) on center for ½ inch thick (12.7 mm) or 16 inches (406 mm) for 5/8–inch–thick (15.9 mm) gypsum board. All cut or exposed edges, including those at wall intersections, shall be sealed as recommended by the manufacturer.
In 2003 the IRC added this section.
R702.4.3 Limitations. Water-resistant gypsum backing board shall not be used in the following locations:
1. Over a vapor retarder in a shower or bathtub compartment.
2. Where there will be direct exposure to water, or in areas subject to continuous high humidity.
In 2006, section 702.4.2 was changed and 702.4.3 was removed.
R702.4.2 Fiber-cement, fiber-mat reinforced cement, glass mat gypsum backers and fiber-reinforced gypsum backers. Fiber-cement, fiber-mat reinforced cement, glass mat gypsum backers or fiber-reinforced gypsum backers in compliance with ASTM C 1288, C 1325, C 1178 or C 1278, respectively, and installed in accordance with manufacturers' recommendations shall be used as backers for wall tile in tub and shower areas and wall panels in shower areas.
It means "Greenboard" can still be used in bathrooms (and other areas), it just cannot be used as a backer for tile in a shower or tub surround. It can be used in areas not subject to direct water exposure (tub/shower surround), and areas of noncontinuous high humidity (bathrooms).
So, can you use "Greenboard"? Probably. It was done that way from the time "Greenboard" was introduced, until IRC 2003 was adopted.
Should you use "Greenboard"? Probably not (unless that is the only option available to you). Even if your area does not follow International Residential Codes, the IRC can be used as a fairly good set of best practices.