Except for flights to/from Europe and domestic flights that don't have any connections with international itinerary, GA will mostly give you WP miles.

From NWA.COM:

Garuda Indonesia

Earning/Redeeming Miles
All Garuda operated flights except those to/from Europe and redemption between Auckland and Brisbane. Domestic Indonesian flights require a connection to/from a Garuda operated international flight in order to be eligible for earning and redeeming.

Destinations
Garuda Indonesia serves destinations in Asia, Europe and Australia

GA also offers the most economical biz class fares for SIN-DPS or SIN-CGK-DPS. However, they only have 1 flight a day non-stop from SIN, the rest of them have to connect in Jakarta.

I usually book myself on GA. Their inflight service is good... food is great, IFE is outdated but FAs are fantastic. Call 800-3-GARUDA and they usually have a good airfares for travellers from the US. Otherwise, book your tickets when you get to Singapore. Just go to a local travel agency and they'll issue your tickets right on the spot. There is no need of advance reservation for economy class travel. For Business class travel, call the GA's 800 number here in the US for that special fares.

The last time I flew on them, I was the only pax in their biz class cabin with 3 FAs assigned to work the cabin.

SIN-DPS is usually on DC-10 or A-330. SIN-CGK are mostly on 737; however, the connection to DPS is either on DC-10/A-330/744 as those flights usually connect in DPS for Australia/Japan.

Give them a try... but keep in mind that they're not SQ or CX so don't expect the level of service to be at par with them. But for the money you spend, it's worth spending.