GRR is normally a marketing package developer offered to attract investors, especially in this soft market when people worries that they are not easy to get tenant.

There are certainly pro/cons in everything offered to you, so choose wisely. For GRR, pro is certainly hassle free and promising rental returns. There's also a catch, normally GRR is offered in form of 3+3+3, or 2+2, or 2+3, etc... the + are at the option of the developer. In another word, after the first 2 or 3 years, if they cannot fetch enough rental they have all the options to stop continuing with the GRR and you'll have to decide quickly whether to sell or to look for a tenant.

With GRR, it also attracts more investor to the same project. Guest what will happen when the GRR matured or terminated? Price war, how severe have to depend on how fast the market can assimilate the units in market... so that comes back to the question of whether the property is in good demand. 

Another observation is, normally projects offering GRR will have a higher selling price compared to other similar properties... so it actually took some of your future capital appreciation to pay you your rental... that means you're paying yourself lah~....

Hence to me, I won't bother too much about GRR, it's the fundamental potential of the property itself that you need to study and that's going to really GUARANTEE you for long term.