The fastest, easiest way to get the five "Gulp!" pills for Marbles.
Guide to Unlock Marbles
Step 1: Unlock Tainted Cain (Spoilers)
If you haven't yet, you'll need to unlock Tainted Cain. To do so, you'll have to have unlocked Cain, defeated Mother (on any character), and unlocked the ascent path. If you've already unlocked the Red Key, you'll also need to drop a trinket off in a boss room before you fight Mom by holding CTRL, but it will actually be easier to do if you haven't unlocked the key yet.
How to get to home
In Depths II, look for the skull rock with the x on it. Blow it up and take the fool card from inside. Make sure you're playing as Cain.
After defeating Mom, grab the polaroid and use the card.
Place the polaroid in the strange door to go to Mausoleum II.
At the end of this extra floor, you'll get Dad's note. This begins an ascent back through all of the levels. Important: If you've already unlocked the red key and did not get one throughout this run, leave a trinket behind in one of the boss rooms prior to Mom. During your ascent, this should turn into a cracked key, which is just a one-time use card slot version of the active item. If you have not unlocked the key yet, you will in the process and can safely ignore this step.
After you get home, go to Mom's room. If you haven't unlocked the red key yet, there will be one in the big purple box in front of her bed.
Go back into the hallway outside Mom's room, and use the key or fragment to open a secret door to the upper left. In this closet is the tainted version of the character you're currently playing as.
Step 2: Create an Unkillable Money Machine
Tainted Cain's main gimmick is that he cannot grab active items. Instead, he must craft his own using pickups. Using this gimmick, we're going to create a very easy money farm. This uses (primarily) common items which you should be able to acquire within a floor or two.
Recipe 1: Gnawed Leaf
To get this, you'll need to grab four pennies, one key, and three bombs. Do not add anything else, including lucky pennies, to your crafting bag. If you do, you can cycle them out with CTRL. The item on the top left will be replaced.
Recipe 2: Midas' Touch
For this, you'll need one key, two hearts (or half-hearts), a key, and four bombs. This will turn enemies to gold if they touch you, including your stone form from gnawed leaf.
Recipe 3: E. Coli
For this, you'll need three coins, three hearts, a key, and a nickel (which has a high chance of spawning if you use Midas Touch to your advantage). When both of these items are combined, you now turn enemies into coin-packed "gold poops" when touched. Crafting three of these will give you the "Oh Crap!" transformation, which restores half a heart of HP when you break the poops, something I heavily recommend, as it will allow you to gild enemies without having to worry about your stone form starting (or at the very least, makes you mistake-proof).
Recipe 4: Portable Slot
You'll be tempted to craft Mom's Pill Bottle. Don't do that. Instead, craft portable slot, the easiest recipe in the game: eight pennies, something you should be in no shortage of by now.
Now the easy part
Turn everyone into gold poop, destroy them, and begin grabbing random items until you get the Breakfast item (or any other HP up). Seven pennies and a nickel should also give you Rainbow Baby, which will give you better odds against bosses, but at this point you don't need to make a conscious effort to craft any specific items. When your HP is at max and you feel confident that a bunch of pills aren't going to make you vulnerable to dying, spam the slot machine and take any pills it drops. It takes a bit, but you should be able to find five gulps within one run if you've unlocked gulp. This should easily unlock Marbles, as well as Gold Razor. It can also be used for any other coin-related unlocks, or to just fill up your donation box for other characters.
Note: You'll need to complete two consecutive victory laps (defeating the Lamb at the end of each run) for Gulp to even appear in the pills pool.