Can I use Steam digital gift cards to unblock a new Steam account?

Seems a no.

https://steamcommunity.com/discussions/forum/7/1500126447383003623/
https://www.reddit.com/r/Steam/comments/83vj9h/does_buying_games_with_redeemed_digital_gift/
https://steamcommunity.com/discussions/forum/1/1693785669850777963/

why not buy steam wallet instead

It's the other way round. Buying a gift card can un-limit an account but it's the account that buys the card not the one that redeems it.

i have 5 accounts like that and i just bought 5 pounds for each just buy valve cards from offgamers or any site selling cards and redeem it into your 2nd account that's all

No, but it would unlock the account that bought those $5 cards.

If you ask me, just load the wallet with $5 using your standard payment method and you'll be fine. And if you're attempting to hide connection between your alts and your main then don't, there are so many obvious leads you won't cover that you can only make it worse for yourself by attempting to fool them.

I unlocked my alt account by adding funds from adv site, where you complete offers for some points which can be converted to Steam wallet

Thank you all for your answers! 😃