Wasps rugby director McGeechan will take charge of his fourth Lions trip, having led them to Australia (1989), New Zealand (1993) and South Africa (1997).
Guscott, who won 65 caps and scored 30 tries for England, started eight out of nine Tests under McGeechan on those three trips, with the Lions recording series victories in Australia and South Africa.
The former Bath star said: "His credentials for the role are unmatched. It is very difficult to to beat a southern hemisphere power on their patch with a scratch team and little preparation.
"But it is also the chance of a lifetime to work with an inspirational coach like Geech.
"I was lucky enough to be selected for the 1989, 1993 and 1997 tours by Geech, and they remain the pinnacle of my playing career.
"Each tour was different, each time he adjusted to the challenge and prepared players brilliantly. We won the Test series on two of those three tours, and I give Geech most of the credit."