Gmail provides us with 15 GB of free storage. If 15 GB storage has been used and google suggests you buy extra space, then you can buy Gmail space by buying the Gmail paid subscription.
But if you cannot pay for a paid subscription that case you can delete the selected big-size files that are not in use.
First, go to your Gmail inbox and search the following. Here will sort 2 MB files to delete.
write hash:attachment larger:2m and press enter
You will get a list of files larger than 2 MB. You can select the useless files and delete them one by one.
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