The four-time All-Pro is reportedly considering retirement. After the Patriots' victory over the Los Angeles Rams in Super Bowl LIII, Gronkowski said he'll take a "week or two" and evaluate his future.
His presence at the team's facility could be related to treatment for the thigh injury he struggled through for much of the 2018 season, Reiss adds.
The three-time Super Bowl champion also visited Boston Children's Hospital, spending time with patients and donating $80,000.
Gronkowski turns 30 in May and carries a nearly $12 million cap hit during the final year of his contract in 2019.