Scenario: 

You have a DoubleClick transaction tag [not items sold type] and want to pass the details of products purchased as one of the custom variables in DoubleClick. The ecommerce transaction [and products] itself is available in the dataLayer for you to use.

We want u1 to be 'product 1 | product 2 | product 3' [if 3 products are sold]. If only one product is sold in a transaction , it should read as 'product 1'.

Use dataLayer to capture products purchased as a custom variable in GTM

Step 1: We need to capture the values [array] of products entered within the ecommerce transaction DL push. 

In the above example, there are two products in the transaction, socks and t-shirt. We want to capture '0044|AA1243544' as the value in this JS variable.

Create a DL variable for transactionProducts like this:

Step 2: (The tricky one) is passing those values to a Custom JS variable. This is where I posted this on GTM Forum and once again, Simo Ahava helped with it.  Let's call this variable {{CJS - productSKU}}

How I understood this JS [thanks to our dev]: This JS uses the from the dataLayer.. It then creates another variable , idString as blank. For each products contained in the transaction, it uses the SKU field available within the array and adds the pipe symbol in between . In the end, it trims the text to remove extra spacing.

You now have this variable which can be used in your DoubleClick custom variables as {{CJS - productSKU}}.

Here's a link to the GTM Forum thread:

https://productforums.google.com/forum/#!topic/tag-manager/Fd1jLT1BgTo;context-place=forum/tag-manager