Rulings
Q1: Does "If the total number of cards in both players' trashes is 20 or more" mean 20 or more cards in either me or my opponent's trashes?
No, it means when the cards in your trash and the cards in your opponent's trash add up to 20 or more cards in total. For example, the condition is met if you have 12 cards in your trash and your opponent has 8 cards in their trash.
Q2: After gaining Rush from this card's On Play effect, is Rush lost if the total cards in both players' trashes is reduced to 19 or fewer cards?
No, "for the turn" effects persist as long as the conditions were met at the time of activation.
Data Sources
EN
Official English Card Listworld.digimoncard.com
Lv.3 Guilmon EX4-006 C Rookie | Virus | Reptile [On Play] If the total number of cards in both players' trashes is 20 or more, this Digimon gains <Rush> for the turn. (This Digimon may attack the turn it was played.)
A
Official English (Asia) Card Listen.digimoncard.com
Lv.3 Guilmon EX4-006 C Rookie | Virus | Reptile Digivolve: 0 from [Gigimon] [On Play] If the total number of cards in both players' trashes is 20 or more, this Digimon gains <Rush> for the turn. (This Digimon may attack the turn it was played.)
W
DCG Wikidigimoncardgame.fandom.com
Lv.3 Guilmon EX4-006 C
Rookie | Virus | Reptile
{{DigivolveFromNames|0|Gigimon}}
[On Play] If the total trashes of both player's add up to 20 or more cards, this Digimon gains <Rush> for the turn.JP
Official Japanese Card Listdigimoncard.com
Lv.3 ギルモン EX4-006 C 成長期 | ウィルス種 | 爬虫類型 【登場時】お互いのトラッシュ合計が20枚以上なら、このターンの間、このデジモンは≪速攻≫（このデジモンは登場したターンでもアタックできる）を得る。