Now Thomas is determined to prove his worth to Gatland ahead of the Six Nations with a top display against Treviso.
"I was disappointed to miss out for Wales but my aim is to get back in the squad," said the 25-year-old.
"I've got to play well, hit some form and prove to Warren I'm over my injury.
"The rehab on my groin has gone well and it's vital I'm fully fit for the next two months. There are crucial games ahead which will dictate how our season goes."
The Ospreys have been hit by a flu bug ahead of their must-win European Cup clash with the Italians at the Liberty Stadium.
Lee Byrne, Filo Tiatia and Andrew and David Bishop have all missed training this week and are fitness doubts for the crunch game in Pool Three.
The Welsh region will also be without stars Gavin Henson, Mike Phillips, Jamie Nutbrown, Duncan Jones, Huw Bennett and Ian Evans through injury.
Ospreys coach Sean Holley said: "We won't name our team until today's (Friday's) deadline because there are several of the players who have a sickness bug.
"These sorts of things tend to come around at this time of the year and we've made sure they've all had the flu jab this week.
"We're delighted to have many of the Welsh players back. They've all been knocking on my door asking to play and most of them will be involved."
IRB World Player of the Year Shane Williams will start despite a cameo appearance for the Barbarians on Wednesday while Wales star Sonny Parker is fit again.