You may get .psd* file of "Sparkling Drink" Photoshop tutorial by making a donation. Put the tutorial title in "Add special instructions to seller" line of PayPal and it will be sent to you shortly.
Select part of the glass which is supposed to be filled in with drink and create a new layer.
Fill it with red/transparent gradient again.
Adjust Linear Burn Blending Mode for the Layer with gradient.