Here is what i have done for solving the same issue, that you are facing.
I have added below image in my image assets,

Now i added below method in my code:
-(UIImage*)drawText:(NSString*)text inImage:(UIImage*)image
{
UIFont *font = [UIFont boldSystemFontOfSize:11];
CGSize size = image.size;
UIGraphicsBeginImageContextWithOptions(size, NO, 0.0f);
[image drawInRect:CGRectMake(0, 0, size.width, size.height)];
CGRect rect = CGRectMake(0, 0, image.size.width, image.size.height);
NSMutableParagraphStyle *paragraphStyle = [[NSParagraphStyle defaultParagraphStyle] mutableCopy];
paragraphStyle.alignment = NSTextAlignmentCenter;
NSDictionary *attributes = @{
NSFontAttributeName : font,
NSParagraphStyleAttributeName : paragraphStyle,
NSForegroundColorAttributeName : [UIColor redColor]
};
CGSize textSize = [text sizeWithAttributes:attributes];
CGRect textRect = CGRectMake((rect.size.width-textSize.width)/2, (rect.size.height-textSize.height)/2 - 2, textSize.width, textSize.height);
[text drawInRect:CGRectIntegral(textRect) withAttributes:attributes];
UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
return newImage;
}
Now, I called this method, while assigning icon to GMSMarker, like this:
marker.icon = [self drawText:@"$33.6" inImage:[UIImage imageNamed:@"icon-marker"]];
It will generate the image icon like below:

Here, I kept the background Image size fixed, as i needed. You can still customize it to adjust it according to text size, as well as multiple lines.
UPDATE
Updated code in Swift:
func drawText(text:NSString, inImage:UIImage) -> UIImage? {
let font = UIFont.systemFont(ofSize: 11)
let size = inImage.size
//UIGraphicsBeginImageContext(size)
let scale = UIScreen.main.scale
UIGraphicsBeginImageContextWithOptions(inImage.size, false, scale)
inImage.draw(in: CGRect(x: 0, y: 0, width: size.width, height: size.height))
let style : NSMutableParagraphStyle = NSMutableParagraphStyle.default.mutableCopy() as! NSMutableParagraphStyle
style.alignment = .center
let attributes:NSDictionary = [ NSAttributedString.Key.font : font, NSAttributedString.Key.paragraphStyle : style, NSAttributedString.Key.foregroundColor : UIColor.black ]
let textSize = text.size(withAttributes: attributes as? [NSAttributedString.Key : Any])
let rect = CGRect(x: 0, y: 0, width: inImage.size.width, height: inImage.size.height)
let textRect = CGRect(x: (rect.size.width - textSize.width)/2, y: (rect.size.height - textSize.height)/2 - 2, width: textSize.width, height: textSize.height)
text.draw(in: textRect.integral, withAttributes: attributes as? [NSAttributedString.Key : Any])
let resultImage = UIGraphicsGetImageFromCurrentImageContext()
UIGraphicsEndImageContext()
return resultImage
}