** Gulfport Energy's GPOR.N shares up 3.5% to $168.50 in premarket trading on Mon as co poised to join S&P SmallCap 600 index .SPCY this week

** Natural-gas weighted E&P firm to replace Select Medical SEM.N in SPCY effective Weds, July 1, S&P Dow Jones Indices said late Fri

** Rehab healthcare operator SEM being acquired by a consortium led by some of its execs, including Executive Chairman and co-founder Robert Ortenzio, and private equity firm Welsh, Carson, Anderson & Stowe

** Stephens analyst Melissa Roberts said in note she expects passive funds will need to purchase 2 mln GPOR shares, implying about 6 days of buying demand

** Oklahoma City, Oklahoma-based GPOR has ~18 mln shares outstanding for $3 bln market cap

** Through Fri close, GPOR shares down ~22% YTD

** 8 of 13 analysts rate the stock "strong buy" or "buy", 5 "hold"; median PT $235.50, per LSEG data