Sample for using WMS sources in Google Maps SDK for iOS. available on github here: https://github.com/Sumbera/WMS_iOS_GoogleMapSDK
Provide your API key in the WMSController.h

• Google Maps for iOS used : 1.7.2 (April 2014)
• used XCode 5.1.1 (April 2014)
• iPad Air, iOS 7.1 (should run in iOS6.0 too)

There are two ways of overlaying WMS in the Google Maps for iOS SDK:

“Method B”: use GMSTileURLConstructor

   // -- method B. WMS tile layer with GMSTileURLConstructor
      GMSTileURLConstructor urls = 
         ^(NSUInteger x, NSUInteger y, NSUInteger z) {
           BBox bbox = bboxFromXYZ(x,y,z);
           NSString *urlKN = 
             [NSString stringWithFormat:@"Your WMS url&BBOX=%f,%f,%f,%f",
                                           bbox.left,
                                           bbox.bottom,
                                           bbox.right,
                                           bbox.top];

          return [NSURL URLWithString:urlKN];
      };

“Method A”: use custom TileLayer derived from GMSTileLayer

1. your derived class from GMSTileLayer (here WMSTileLayer.h) will receive tile request

    -(void)requestTileForX:(NSUInteger)x 
                                    y:(NSUInteger)y
                                 zoom:(NSUInteger)z 
                             receiver:(id<GMSTileReceiver>)receiver
   

2. WMSTileLayer first checks for cached tile and if found calls :

     [self drawTileAtX:x y:y zoom:z Url:urlStr Receiver:receiver] ;
   

3. if tile is not cached we download it, save it to the file system (using MD5 hash) and call to draw it

     [data  writeToFile: filePath  atomically:YES];
     [self drawTileAtX:x y: y zoom: z Url:urlStr Receiver:receiver] ;
   

4. drawTileAtX is very simple:

     -(void) drawTileAtX: (NSUInteger) x 
                                  y:(NSUInteger) y
                               zoom:(NSUInteger)zoom
                                Url:(NSString*) url
                           Receiver: (id<GMSTileReceiver>) receiver {
          UIImage             *image   = TileLoad(url,NO); 
          [receiver receiveTileWithX:x y:y zoom:zoom image:image]; 
     }
   

}

both ways are used in this sample.