Gregg, who earlier this summer placed Andy Crawford and Kyle Moran also on the list, admits he needs to off load some players before he can bring anyone in.
"I have a good squad, but I feel we still need to strengthen my squad in certain areas, but the only way I can free some money up is by trying to sell some players, so that's why I have had to make the tough decision and place both Marty and Ciaran on the transfer list," said Gregg.
"I know there are a few clubs interested in both players so something might happen sooner rather than later.
"My two main priorities are a striker and a left winger, I feel I have cover in virtually every other positions.
"I know I have my defence sorted, I feel that the signing of James Quigley means I'm happy with my midfield. But I think we need to sign a striker and a left sided player.
"However I said at the start of the season that I didn't want to bring in anyone who isn't going to compete for places.
"This season I have told my players that they aren't competing with themselves, they are competing to get chosen, because I feel there are enough good players in our squad, apart from up front."
Gregg is also worried that a number of his senior players won't be ready for the start of the new look Premiership season next month.
With only 17 days before 'Stute travel to Glenavon for their league opener, the likes of Ruairi Boyle, Declan Divin, Mark Scoltock, Ryan McCreadie and new signings Quigley, Paul Brown, Paddy McLaughlin and Sean Roddy, who joined the club from Finn Harps last week, have missed some parts of pre-season training.
"I'm four weeks into pre-season training and both Decky and Ryan haven't taken part and that's a worry for me," declared Gregg.
"Someone asked me after the Tamworth game, 'why did I play Paul Brown up front on his own', it was because I didn't have another striker available, but I'm still confident that we'll get there."
The Institute gaffer also admitted that Saturday's 1-1 draw at Glebe Rangers, was all about having a look at a few trialists, with Calvin Mooney, who incidentally equalised, Damian Whitehead, Jason Blackburn and John Kelly all getting a run out.
"Saturday's match allowed me to have a good look at players and in particular some of our young lads to see if they could make the step up.
"We had a good work out and I was pleased that a lot of my senior players got more minutes under their belts."
Meanwhile 'Stute travel to Ferney Park, this Saturday as they step up their pre-season training, when they take on Ballinamallard (KO 3pm).