Unless you get super lucky, there's not much chance of catching up to him in a Gardens race if you are taking the time to almost drain the Talismans and Sages. Furthermore with no +buy or gainer it will take 8 turns to drain the Gardens, giving your opponent enough time to grab a couple Provinces and steal the win if he isn't going for Gardens.
Ah, but you wouldn't be the one doing the "catching up". Your Talismans are gaining multiple cards a turn so your VP per Gardens would quickly increase beyond your opponent's. If he's buying Gardens to keep them away from you, then he's clogging his deck with lower value green cards (if he were going Gardens as the primary strategy, he'd probably be grabbing Talismans too, so it'd be a mirror game and strategy discussion is moot). And if he manages to "grab a couple of Provinces" it won't be enough to match your stack of 4+ VP Gardens. And the longer he tries to extend the game to get more Provinces, the more valuable your Gardens become. Done right, it becomes a race that he will
never be able to win, even if he gets all 8 Provinces.
Sage opening is helpful, but I wouldn't worry too much about massing Sages unless you have a $3 turn, which should be rare with a 9/3 treasure/green ratio after your first shuffle (I'm counting the Sage as a Talisman, since that's the only thing that it would draw at that point).
The complicated thing with Talisman is when do you pass up multiple cards to get a Gardens? It's a tough choice when you draw four Talismans in the same hand to sacrifice four extra cards just to buy the Gardens, but if you empty stacks too quickly, the game could end while you only hold a couple of 4-5 VP Gardens. As a general rule, once I start greening, I buy Gardens with 0 or 1 Talismans in hand, and other cards with 2+.