If you a talking about Java then answer is: they will run in parallel. Garbage collection has nothing to do with the thread management.
You can see it with this sample code:
public class LostThread {
public static void main(String[] args) {
for (int i = 0; i < 10; i++) {
final int value = i;
new Thread(new Runnable() {
@Override
public void run() {
while (true) {
try {
System.out.println(value);
Thread.sleep(1000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
}).start();
}
}
}
Main finishes after ten threads are created, but they are still running.