Memory Grants Pending Counter indicates the total number of processes waiting for a workspace memory grant. The recommended Memory Grants Pending value is zero, meaning no processes are waiting for the memory. Anything above 0 indicates that there are processes waiting for memory and that you should probably increase the memory allocated to SQL Server.
Here is the query that will return Memory Grants Pending Counter.
SELECT object_name, counter_name, cntr_value
WHERE [object_name] LIKE '%Memory Manager%'
AND [counter_name] = 'Memory Grants Pending'