With the 'Fins traditional, corporate-style, organizational power structure, I really don't think it matters whether or not Gase realizes that Tannehill "is not the guy". Gase probably takes what Grier gives him in terms of player personnel. Grier answers to Tannenbaum (Exec VP), who most likely has final say over player and coaching personnel (and likely uses that authority unwisely, just as he's done everywhere else he's been).
This is the way most teams in the NFL operate. The Pats' genuinely collaborative decision-making is extremely unusual.
My guess is that, if Gase alone is fired after three seasons, it's mostly scapegoating. OTOH, if Tannenbaum is fired, it may indicate that Ross and Tom Garfinkel (the CEO) have started to wise up a bit.