When entering the Gauntlet, the game lets us choose a previously created deck from our collection, but there is no immediate option to make a new one in compliance with this weeks specific deckbuilding rules.
Ofc this didn't catch anyones eye last week, because the first gauntlet rules were exactly identical to constructed, but today all decks have a big red "invalid" checkmark on top of them.

What will happen from there worst case?

• Go to the collection (1 extra click)
• Start making a new deck (1 extra click)
• Notice you didn't look up the deckbuilding rules for this week and go back to Gauntlet (2 extra clicks; 3 if the started deck is still invalid for constructed)
• Read the description and realise it doesn't specify the rules at all beyond the term "Singleton deck"
• Hopefully don't be lost and quit the game for good at this point, but remember that at least there's patch notes and announcements available ingame by now (2 extra clicks)
• Read that description where it eventually says "using a deck with no duplicates". Go back to the deckbuilder (3 or 4 extra clicks depending on if you discarded or saved your previously started deck)
• Finish your deck and finally start the Gauntlet (2 extra clicks)

That's about a dozen unnecessary clicks and a fair amount of headache that should be prevented as follows:

• 1. Put a "New Deck" button into the Gauntlet tab, that leads to a separate deckbuilder version particular to every weeks specific ruleset. (only 1 extra click)
• 2. This deckbuilder gives an invalid warning based on those rules instead of the normal constructed ones.
• 3. Put the most detailed description of the ruleset somewhere into the Gauntlet tab so we don't have to go look for it outside.

Yes I did exaggerate a bit, but I can see the repetition of this process every week to cause major annoyance in the future.

edit: additional remarks from answers below

the new deck should be saved into your collection so when the gauntlet returns you can go back to the deck (by u/mvhh2000)