What is the syntax for querying a spatial column?
I think you want MBRContains
I have an example from a previous post : Less restrictive query return less result due to simple removing one additional constraint
Give it a Try !!!
UPDATE 2013-05-08 17:05 EDT
You have this:
SELECT * 
FROM tb_gps 
WHERE MBRWITHIN(pt, MULTIPOINT(52.3641917981 4.87673850395, 52.3821782019 4.90619949605))

Put the MULTIPOINT in quotes
SELECT * 
FROM tb_gps 
WHERE MBRWITHIN(pt, 'MULTIPOINT(52.3641917981 4.87673850395, 52.3821782019 4.90619949605)')

I did this in my example
UPDATE 2013-05-08 17:39 EDT
If MULTIPOINT is not working for you, try using POLYGON
SELECT * 
FROM tb_gps 
WHERE
    IF(CONTAINS(GeoFromText('POLYGON(52.3641917981 4.87673850395,
    4.87673850395 4.90619949605,4.90619949605 52.3821782019,
    52.3821782019 52.3641917981)'pt),1,0)
;

You know something? It just dawned on me. You have the parameters backwards.
You have
SELECT * 
FROM tb_gps 
WHERE MBRWITHIN(pt,'MULTIPOINT(52.3641917981 4.87673850395, 52.3821782019 4.90619949605)')

Try reversing the parameters:
SELECT * 
FROM tb_gps 
WHERE MBRWITHIN('MULTIPOINT(52.3641917981 4.87673850395, 52.3821782019 4.90619949605)',pt)

Give it a Try !!!