GC_FOR_MALLOC means that the GC was triggered because there wasn't enough memory left on the heap to perform an allocation. Might be triggered when new objects are being created. 

GC_EXPLICIT means that the garbage collector has been explicitly asked to collect, instead of being triggered by high water marks in the heap. Happens all over the place, but most likely when a thread is being killed or when a binder communication is taken down. 

There are a few others as well: 

GC_CONCURRENT Triggered when the heap has reached a certain amount of objects to collect. 

GC_EXTERNAL_ALLOC means that the the VM is trying to reduce the amount of memory used for collectable objects, to make room for more non-collectable. 

typedefenum{ 
    /* Not enough space for an "ordinary" Object to be allocated. */ 
    GC_FOR_MALLOC, 
    /* Automatic GC triggered by exceeding a heap occupancy threshold. */ 
    GC_CONCURRENT, 
    /* Explicit GC via Runtime.gc(), VMRuntime.gc(), or SIGUSR1. */ 
    GC_EXPLICIT, 
    /* GC to try to reduce heap footprint to allow more non-GC'ed memory. */ 
    GC_EXTERNAL_ALLOC, 
    /* GC to dump heap contents to a file, only used under WITH_HPROF */ 
    GC_HPROF_DUMP_HEAP 
}GcReason; 

GC_EXTERNAL_ALLOC freed 297K, 49% free 3411K/6663K, external 24870K/26260K, paused 83ms 

前面Free的内存是VM中java使用的内存，external是指VM中通过JNI中Native的类中的malloc分配出的内存，例如Bitmap和一些Cursor都是这么分配的。 
在Davilk中，给一个程序分配的内存根据机型厂商的不同，而不同，现在的大部分的是32M了，而在VM内部会把这些内存分成java使用的内存和 Native使用的内存，它们之间是不能共享的，就是说当你的Native内存用完了，现在Java又有空闲的内存，这时Native会重新像VM申请，而不是直接使用java的。 
例如上边的例子 
free 3411K/6663K和external 24870K/26260K 
如果这时需要创建一个2M的Bitmap，Native现有内存26260-24870=1390K<2048k,因此他就会向Vm申请内存，虽然java空闲的内存是 
6663-3411=3252>2048,但这部分内存Native是不能使用。 
但是你现在去申请2M的Native内存，VM会告诉你无法分配的，因为现在已使用的内存已经接近峰值了32M(26260+6663=32923 )，所以现在就会成force close 报OOM。 
所以现在我们要检查我们的native内存的使用情况来避免OOM。 

别的资料：http://stackoverflow.com/questions/4525743/what-are-the-paused-values-in-gc-concurrent-log-messages