This is quite rare but in case it happens, it's important to distinguish between different scenarios:
Poster and frame ordered from different suppliers
If you ordered the poster and the frame from different suppliers (e.g. the frame from Gelato and the poster somewhere else or vice-versa), you have to consider that Gelato applies centimeters to inches (and vice-versa) automatic conversion depending on where the order is shipped to. If the order is going to the US or Canada, it will be processed in inches; if it's going to the rest of the world, it will be processed in centimeters.
We always recommend ordering both the poster and the matching frame from Gelato to avoid such cases.
The poster and frame were ordered from Gelato
If you ordered both the poster and the frame from Gelato, there are a few checks to perform:
Check if the poster fits from the back (see picture below) - instructions here. Sometimes, customers are testing if the poster fits into the frame by laying the poster on the front of the frame while the back of the frame has actually more space than the front.
If the poster doesn't fit from the back, measure the poster length and width and contact Gelato Support Team.