I wanted to ask if the summarisetogene() function would result in sum of transcript level expressions for a gene if both were normalised using the median ratio's method. I know that TPM values follow this pattern but I wanted to know about this as the sum of transcript level expression differed slightly from the gene level expression.

Thank you for your help!

The count you get with default settings is just the sum of the txp counts. See Soneson, Love, Robinson (2015) for details.