ATCT LCL CTLR CLRED AN A320 FOR TKOF WHILE A B757 WAS ON SHORT FINAL. THE TWR CTLR TOLD THE B757 TO GAR; BUT DID NOT ASSIGN A HDG OR ALT. FREQ WAS CONGESTED AND THE CTLR FINALLY GAVE A HDG AND ALT ASSIGNMENT AND RPTR THINKS THAT THIS SHOULD HAVE BEEN DONE SOONER OR PRE ASSIGNED WHEN A GAR MIGHT BE IMMINENT.

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MSP TWR PUT A320 IN POS AND HOLD WITH LIGHT TWIN ON TKOF ROLL AND OUR ACFT INSIDE 3 MI. A320 STARTED TKOF ROLL IMMEDIATELY WHEN CLRED; BUT IT WAS TOO LATE (OUR ACFT AT APPROX 500 FT) AND TWR SAID 'GAR' TO US WITH NO FURTHER INSTRUCTIONS. TWR FREQ WAS BUSY; AND HDG/ALT INSTRUCTIONS (IF ISSUED) WERE BLOCKED. WE MAINTAINED RWY HDG INITIALLY AND BEGAN CLB TO 4000 FT (ILS MISSED APCH ALT). WHEN WE GOT A WORD IN EDGEWISE AND ASKED FOR HDG/ALT; TWR SAID WE 'COULD' MAINTAIN 3000 FT (WE WERE PASSING 3400 FT) AND ASKED IF WE COULD TURN TO A 240 DEG HDG (WE COULD). ALTHOUGH WE SAW THE GAR COMING EARLY; THE TWR CTLR SEEMED UNPREPARED; AND CONFUSION REIGNED. ALTHOUGH I UNDERSTAND THE NEED TO GET DEPS OUT IN BTWN ARRS; PERHAPS IN A SIT WHERE A GAR IS A DISTINCT POSSIBILITY; CONTINGENT GAR INSTRUCTIONS SHOULD BE ISSUED TO THE ACFT ON THE VISUAL APCH -- JUST IN CASE. ALTHOUGH WE NEVER GOT ANY TCASII ADVISORIES; IT APPEARED (AND SOUNDED LIKE) SEPARATION BTWN US AND THE DEPARTING A320 WAS RAPIDLY BECOMING AN ISSUE. A PRE-ASSIGNED HDG FOR US (IN THE EVENT OF A GAR) WOULD HAVE SOLVED THE PROB.