Here is the relevant text from that web site.

""PRUNING" ANTENNAS
The unit is shirt-pocket sized so you can use it up the mast. To check a
vertical 1/4 wave (for example) disconnect the feeder, and short the
antenna with a bit of wire about 5-10cm long. Twist the wire to form a
single turn loop and check this loop with the GDO as above. If the ANT
is too short the DIP will be higher than the frequency you want."

W1MCE

Max wrote:
Hello Frank


I was wondering how I can determine
the resonance frequency of an antenna
(diple) by using a grid-dip meter (GDO)

I tried to search for it on Google with
poor results.


Try http://w1.859.telia.com/~u85920178/use/gdo.htm

Regards Max