If I use Guzzlord GX's "Eat Sloppily" attack and discard a Beast Energy {*} card, would it get attached to the Guzzlord GX, or would it go straight to the Lost Zone because it was discarded?
When "discarding the top 5 cards of your deck", any revealed Energy cards get attached to Guzzlord GX immediately, before they reach the Discard Pile. Therefore, Beast Energy {*} gets attached to Guzzlord GX rather than going to the Discard Pile or Lost Zone.
Source: TPCi Rules Team (2018-05-31)

Relevant cards

Note: All applicable cards may not be listed below.
SM4_105
SM4_116
SM4_63
SM4_63a
SMA_SV71
SMA_SV71
SM6_117