Gundestup can not be used as a reliable source.
The silverworking techniques used in the cauldron are unknown from the Celtic world, but are consistent with the renowned Thracian sheet-silver tradition; the scenes depicted are not distinctively Thracian, but certain elements of composition, decorative motifs and illustrated items (such as the shoelaces on the "Cernunnos" figure) identify it as Thracian work (Bergquist, A. K. & Taylor, T. F. (1987) "The origin of the Gundestrup cauldron" in Antiquity Vol. 61, 1987. pp. 10-24).
Bergquist and Taylor propose manufacture by a Thracian craftsman, possibly commissioned by the Celtic Scordisci and fallen into the hands of the Cimbri who invaded the Middle lower Danube in 120 BC.
Here is an example of a non-v-neck tunic found in southern Germany...http://www.euratlas.com/Atlas/germany_r ... _celt.html
The statue seems to be weareing sandal boots that have the trousers tucked into them.
As for the ankle ties, if one wears boots, the trousers are tucked into the boot, no need for ankle ties. Another suggestion is to tie ones shoes around the ankle, this is more comfortable than having leather tied around the bare skin. Another practical reason to tie at the ankle is to prevent fleas and ticks to crawl up your legs.