Fresh off his MotoGP race debut at Assen last weekend, American rider Gerloff lapped the Donington circuit in 1m27.52s at the death of the second practice session, overhauling Rea's FP1 benchmark of 1m27.841s.
Gerloff and Rea were the only two times to break the 1m28s barrier across the two sessions, with the latter's FP2 lap of 1m27.853s fractionally slower than the time with which he topped the first practice earlier in the day.
Toprak Razgatlioglu, who will remain with the factory Yamaha team for the next two seasons, ended up third overall with a last-gasp effort of 1m28.048s. The Turkish rider was Rea's top challenger in FP1 earlier, ending second in that session with a 1m28.105s.
Alex Lowes established an early 1-2 for Kawasaki in second practice but his final time of 1m28.131s was ultimately good for only fourth overall, six tenths off the pace of Gerloff's Yamaha.
Former MotoGP rider Scott Redding placed the factory Ducati fifth in the combined timesheets after setting his best time of 1m28.150s with less than two minutes to remain in FP2.
BMW ace Tom Sykes made it four different manufacturers in the top six with a 1m28.202s flyer that was set in the morning session, with the 2013 champion unable to improve on that effort in second practice later in the afternoon.
Michael Ruben Rinaldi, who scored a double win at Misano last time around, finished just two tenths adrift of his Ducati teammate Redding in seventh, as Leon Haslam was Honda's top runner in eighth.
Puccetti Kawasaki rider Lucas Mahias was classified ninth, while Michael van der Mark completed the top 10 on the second of the factory BMWs.
GoEleven rider Chaz Davies could only manage the 12th quickest time behind the factory Yamaha of Andrea Locatelli, finishing more than 1.5s off the pace in FP2.
Honda rider Alvaro Bautista was also unusually off the pace, finishing 14th and 15th in the two sessions with a best time of 1m29.568s.
Elsewhere, Gerloff's teammate Kohta Nozane has been declared unfit for the weekend after suffering a hairline fracture to his right hand's middle finger during testing at the Navarra Circuit. The Japanese rider completed just four laps in FP1 before sitting out the rest of the session.
Ahead of WSBK's first visit to Donington in two years, Pedercini Racing and Samuele Cavalieri announced that they've split with immediate effect. Luke Mossey has been called in as his replacement at Donington Park but has not been confirmed for the remainder of the year. The British rider ended the day 17th quickest in FP2.
FP1 results:
| Cla | # | Rider | Bike | Time | Gap |
|---|---|---|---|---|---|
| 1 | 1 |  Jonathan Rea | Kawasaki | 1'27.841 | |
| 2 | 54 |  Toprak Razgatlioglu | Yamaha | 1'28.105 | 0.264 |
| 3 | 66 | Tom Sykes | BMW | 1'28.202 | 0.361 |
| 4 | 21 |  Michael Ruben Rinaldi | Ducati | 1'28.560 | 0.719 |
| 5 | 31 |  Garrett Gerloff | Yamaha | 1'28.681 | 0.840 |
| 6 | 45 | Scott Redding | Ducati | 1'28.814 | 0.973 |
| 7 | 91 | Leon Haslam | Honda | 1'28.843 | 1.002 |
| 8 | 22 | Alex Lowes | Kawasaki | 1'29.028 | 1.187 |
| 9 | 60 |  Michael van der Mark | BMW | 1'29.104 | 1.263 |
| 10 | 7 | Chaz Davies | Ducati | 1'29.172 | 1.331 |
| View full results | |||||
FP2 results:
| Cla | # | Rider | Bike | Time | Gap |
|---|---|---|---|---|---|
| 1 | 31 | Garrett Gerloff | Yamaha | 1'27.524 | |
| 2 | 1 | Jonathan Rea | Kawasaki | 1'27.853 | 0.329 |
| 3 | 54 | Toprak Razgatlioglu | Yamaha | 1'28.048 | 0.524 |
| 4 | 22 | Alex Lowes | Kawasaki | 1'28.131 | 0.607 |
| 5 | 45 | Scott Redding | Ducati | 1'28.150 | 0.626 |
| 6 | 21 | Michael Ruben Rinaldi | Ducati | 1'28.324 | 0.800 |
| 7 | 91 | Leon Haslam | Honda | 1'28.395 | 0.871 |
| 8 | 44 |  Lucas Mahias | Kawasaki | 1'28.462 | 0.938 |
| 9 | 60 | Michael van der Mark | BMW | 1'28.549 | 1.025 |
| 10 | 55 | Andrea Locatelli | Yamaha | 1'28.594 | 1.070 |
| View full results | |||||
