Rulings
Q1: Is "Digivolve While you have 2 or fewer security cards, Gabumon: Cost 3" a digivolution requirement added by an effect while I have 2 or fewer security cards?
No, this is a standard digivolution requirement for a Digimon card. It references the number of your security cards, but it's not an effect.
Q2: Can I use Blast Digivolve to digivolve my Gabumon into this card in the hand when I have 2 or fewer security cards and an opponent's Digimon attacks?
Yes, you can. Digivolution is possible as long as you have 2 or fewer security cards when you use Blast Digivolve.
Q3: If an opponent's Digimon attacks when I have 2 or fewer security cards, then an effect adds a card to my security stack so my security stack has 3 or more cards when I use Blast Digivolve, can my Gabumon digivolve into this card in the hand?
No, it can't. Digivolution from Gabumon isn't possible because the digivolution requirements aren't met when you use Blast Digivolve.
Q4: If an opponent's Digimon attacks when I have 3 security cards, then an effect removes a card from my security stack so my security stack has 2 or fewer cards when I use Blast Digivolve, can my Gabumon digivolve into this card in the hand?
Yes, you can. Digivolution from Gabumon is possible as long as the digivolution requirements are met when you use Blast Digivolve.
Q5: If I use an effect such as P-103 Offense Training's Delay that digivolves a Digimon, can my Gabumon digivolve into this card in the hand when I have 2 or fewer security cards?
Yes, you can.
Data Sources
Lv.7 Gabumon - Bond of Friendship ACE LM-022 P Mega | Unknown | Unknown [Digivolve] While you have 2 or fewer security cards, [Gabumon]: Cost 3 [Hand] [Counter] <Blast Digivolve> (Your Digimon may digivolve into this card without paying the cost.) [On Play] [When Digivolving] Return 2 of your opponent's Digimon with as many or fewer digivolution cards as this Digimon to the bottom of the deck. [When Attacking] [Once Per Turn] If you have a Tamer, this Digimon unsuspends. --- Inherited: <Overflow (-5)> (As this card moves from the field or under a card to an area other than those, lose 5 memory.)
Lv.7 Gabumon - Bond of Friendship ACE LM-022 P Mega | Unknown | Unknown [Digivolve] While you have 2 or fewer security cards, [Gabumon]: Cost 3 [Hand] [Counter] <Blast Digivolve> (One of your Digimon may digivolve into this card without paying the cost.) [On Play] [When Digivolving] Return 2 of your opponent's Digimon with as many or less digivolution cards as this Digimon to the bottom of the deck. [When Attacking] [Once Per Turn] If you have a Tamer, unsuspend this Digimon. --- Inherited: <Overflow (-5)> (When this card would move from the battle area or under a card to another area, lose 5 memory.)
Lv.7 Gabumon - Bond of Friendship ACE LM-022 P Mega | Unknown | Unknown [Digivolve] While you have 2 or fewer security cards, [Gabumon]: Cost 3 [Hand] [Counter] <Blast Digivolve> (One of your Digimon may digivolve into this card without paying the cost.) [On Play] [When Digivolving] Return 2 of your opponent's Digimon with equal or fewer digivolution cards than this Digimon to the bottom of the deck. [When Attacking] [Once Per Turn] If you have a Tamer, unsuspend this Digimon. --- Inherited: <Overflow (-5)> (As this card moves from the battle area or under a card to another area, lose 5 memory.)
Lv.7 ガブモン -友情の絆-ACE LM-022 P 究極体 | 不明 | 不明 自分のセキュリティが2枚以下の間、「ガブモン」:コスト3 ［手札］【カウンター】≪ブラスト進化≫（自分のデジモンをこのカードにコストを支払わずに進化できる） 【登場時】【進化時】進化元の枚数がこのデジモン以下の相手のデジモン2体をデッキの下に戻す。 【アタック時】[ターンに1回]自分のテイマーがいるなら、このデジモンをアクティブにする。 --- Inherited: ≪オーバーフロー《-5》≫（バトルエリアかカードの下から、それ以外の場所に送られる場合、メモリー-5）