GMail crashes:

 ruby -r net/imap -e 'p Net::IMAP.new("imap.gmail.com", 993, ssl: true)'
Traceback (most recent call last):
	5: from -e:1:in `<main>'
	4: from -e:1:in `new'
	3: from /usr/lib/ruby/2.5.0/net/imap.rb:1092:in `initialize'
	2: from /usr/lib/ruby/2.5.0/net/imap.rb:1531:in `start_tls_session'
	1: from /usr/lib/ruby/2.5.0/net/protocol.rb:44:in `ssl_socket_connect'
/usr/lib/ruby/2.5.0/net/protocol.rb:44:in `connect_nonblock': SSL_connect returned=1 errno=0 state=error: certificate verify failed (self signed certificate) (OpenSSL::SSL::SSLError)


Canonical's imap server seems to work okay:
# ruby -r net/imap -e 'p Net::IMAP.new("mail.canonical.com", 993, ssl: true)'
#<Net::IMAP:0x000055fc83a42600 @mon_owner=nil, @mon_count=0, [...]

openssl itself to gmail seems fine:

# openssl s_client -verify 3 -CApath /etc/ssl/certs -connect imap.gmail.com:993
verify depth is 3
CONNECTED(00000006)
depth=2 OU = GlobalSign Root CA - R2, O = GlobalSign, CN = GlobalSign
verify return:1
depth=1 C = US, O = Google Trust Services, CN = Google Internet Authority G3
verify return:1
depth=0 C = US, ST = California, L = Mountain View, O = Google LLC, CN = imap.gmail.com
verify return:1
---
Certificate chain
 0 s:C = US, ST = California, L = Mountain View, O = Google LLC, CN = imap.gmail.com
   i:C = US, O = Google Trust Services, CN = Google Internet Authority G3
 1 s:C = US, O = Google Trust Services, CN = Google Internet Authority G3
   i:OU = GlobalSign Root CA - R2, O = GlobalSign, CN = GlobalSign
---
Server certificate
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
subject=C = US, ST = California, L = Mountain View, O = Google LLC, CN = imap.gmail.com

issuer=C = US, O = Google Trust Services, CN = Google Internet Authority G3

---
No client certificate CA names sent
Peer signing digest: SHA256
Peer signature type: RSA-PSS
Server Temp Key: X25519, 253 bits
---
SSL handshake has read 2954 bytes and written 401 bytes
Verification: OK
---
New, TLSv1.2, Cipher is ECDHE-RSA-CHACHA20-POLY1305
Server public key is 2048 bit
Secure Renegotiation IS supported
Compression: NONE
Expansion: NONE
No ALPN negotiated
SSL-Session:
    Protocol  : TLSv1.2
    Cipher    : ECDHE-RSA-CHACHA20-POLY1305
    Session-ID: D6976DE28566AC25ECB00243764949F6C8A0683513D94E52AC4064EE84EB1F3A
    Session-ID-ctx: 
    Master-Key: EAA832FA5CF7ACCEF650135BA9A60F0A1C32C5C0E7AA25EAE359CACFE3F6611387217F02767169862037C0971B5A7405
    PSK identity: None
    PSK identity hint: None
    SRP username: None
    TLS session ticket lifetime hint: 100800 (seconds)
    TLS session ticket:
    0000 - 00 c8 62 ba f3 f4 e5 e9-06 0f 2e 57 a4 8b 55 8d   ..b........W..U.
    0010 - 3b c8 2f 3e 63 33 27 e4-27 3f 89 f1 a7 9b 92 0e   ;./>c3'.'?......
    0020 - b1 ef 71 58 b1 69 93 a0-1d 8d 1a ed 69 61 f2 31   ..qX.i......ia.1
    0030 - f2 c2 96 af d5 d4 7b 80-65 68 37 29 7a 96 3e af   ......{.eh7)z.>.
    0040 - 71 e6 b7 fb 8c 65 a9 24-af 46 51 e8 32 db ee d0   q....e.$.FQ.2...
    0050 - 29 97 a9 cc 51 bc e6 93-91 b8 3d 1e 72 12 6c e1   )...Q.....=.r.l.
    0060 - c8 c1 b4 f2 2c bf ae 07-27 34 17 b2 b7 85 e3 d7   ....,...'4......
    0070 - 19 8c 21 66 b2 d4 84 d2-81 b4 fe 96 48 5c 0b 76   ..!f........H\.v
    0080 - 2c 74 b2 dd 12 8f 42 5b-7d a2 14 1b 36 67 3d 50   ,t....B[}...6g=P
    0090 - b0 71 a8 5a c3 e4 e3 e4-e2 f1 cc f5 89 83 2a 7e   .q.Z..........*~
    00a0 - 88 d8 c1 1d 9a 0a 45 c6-f5 57 11 55 d1 c8 57 2c   ......E..W.U..W,
    00b0 - 38 bd eb 61 5e 1b 30 25-b6 ae 7e 60 0a 38 9c ce   8..a^.0%..~`.8..
    00c0 - ff 74 16 42 0c cd e9 a2-7f b2 f2 8b 16 a7 b5 13   .t.B............
    00d0 - 3d cd a0 c9 16 56 a3 87-bb 36                     =....V...6

    Start Time: 1545427402
    Timeout   : 7200 (sec)
    Verify return code: 0 (ok)
    Extended master secret: yes
---
* OK Gimap ready for requests from 71.34.111.116 v142mb720826995oif
DONE

Thanks