Gilliam was moved from RT to LT in the offseason, and the hope was that with his athletic talent and work ethic he would take over the position and be a bargain there for a couple of years.
A few things happened that derailed that plan:
1) Ifedi was moved RG, leaving an open competition at RT.
2) Webb, the front runner to take over at RT, fought injuries, was ineffective once he made it on the field, and was ultimately cut.
3) Sowell impressed coaches in practice (I'm guessing Cable liked his toughness/aggression/physicality), and in an effort to "get the five best bodies on the field," Sowell took over at LT and Gilliam was thrown in to the competition at RT (after spending the offseason preparing to play LT).
4) The entire line including Gilliam struggled (it should be noted that Gilliam was playing next to a rookie).
5) Sowell was injured and Fant took over at LT. Fant was inconsistent and had some major struggles, but impressed with his raw tools and his approach to learning.
6) Gilliam failed to show enough toughness/aggression/physicality (this is typically more important at RT than at LT, where "finesse" players are often successful due to their ability to pass block against speedy edge rushers), and he was benched in favor of Sowell.
7) After a short stint back in the lineup, Sowell proved to be less effective than Gilliam despite having the desired traits, and Gilliam was again put back in the starting lineup. According to Gilliam he made a change in his approach, and focused on being more tough/aggressive physical, which helped him get his starting position back.
I think Gilliam still has the potential to be a solid starter. The tools and the mentality are there. At the very least I think he can provide a lot of value as cheap backup swing tackle.
I think the organization believes in Fant's future at LT and I would be mildly surprised if they signed another LT to a long term deal. A one year "go for a ring" deal for a vet with proven experience in a zone blocking scheme is possible, but I'm not counting on it. Also, as we saw with Webb and Sowell, band-aid vets often do not pan out.
Barring a big signing, I'd like to see Gilliam thrown back in the competition at LT, along with Fant and Odihambo (who actually looked okay in his brief stint at the position). All three are hard workers with talent. If that competition results in a serviceable starter and a decent backup, that's a big win financially.
At right tackle, if they aren't planning on moving Ifedi outside (I doubt they are), I would love to see the Seahawks go after Ricky Wagner, the RT from Baltimore. Wagner is only 27, has the gritty attitude that Cable loves, and was an above-average starter in a Zone Blocking scheme. He would likely cost $7M+ apy but barring a major drop-off he would solidify the RT position for 5+ years.