The 18-year-old centre back from Ireland has come through the youth system at Ewood Park and played regularly in Rovers' reserve side last season.
He has represented Republic of Ireland at Under-19 level and has played for Blackburn's first team during pre-season this summer.
Tranmere boss John Barnes said: "I'm delighted to be able to bring Gavin to Tranmere on loan.
"He is another young, talented defender who will improve our squad.
"I'd like to thank Blackburn Rovers manager Sam Allardyce for allowing Gavin to join us on loan."