Collection was modified; enumeration operation may not execute

Try to read a clone of your collection

foreach (GMapMarker m in Markers.Copy())
{
   ...
}

this will create a new copy of your collection that will not be affected by another thread but may cause a performance issue in case of huge collection.

So I think it will be better if you locked the collection while reading and writing processes.

This is a pretty common mistake - modifying a collection whilst iterating it using foreach, keep in mind that foreach uses readonly IEnumerator instance.

Try to loop through the collection using for() with an extra index check so if the index is out of bounds you would be able to apply additional logic to handle it. You can also use LINQ's Count() as another loop exit condition by evaluating the Count value each time if the underlying enumeration does not implement ICollection:

If Markers implements IColletion - lock on SyncRoot:

lock (Markers.SyncRoot)

Use for():

for (int index = 0; index < Markers.Count(); index++)
{
    if (Markers>= Markers.Count())
    {
       // TODO: handle this case to avoid run time exception
    }
}

You might find this post useful: How do foreach loops work in C#?

You need to lock both on the reading and the writing side. Otherwise one of the threads will not know about the lock and will try to read/modify the collection, while the other is modifying/reading (respectively) with the lock held